lib/serialize.c: cleanup -Wformat-signedness warning.

lib/serialize.c: cleanup -Wformat-signedness warning.

serialize.c: In function 'mc_serialize_str':
serialize.c:116:34: error: format '%zd' expects argument of type 'signed size_t', but argument 3 has type 'size_t {aka long unsigned int}' [-Werror=format=]
     return g_strdup_printf ("%c%zd" SRLZ_DELIM_S "%s", prefix, strlen (data), data);
                                  ^
serialize.c:130:19: error: format '%zd' expects argument of type 'signed size_t', but argument 5 has type 'size_t {aka long unsigned int}' [-Werror=format=]
 #define FUNC_NAME "mc_serialize_str()"
                   ^
serialize.c:175:22: note: in expansion of macro 'FUNC_NAME'
                      FUNC_NAME
                      ^~~~~~~~~
serialize.c:130:19: error: format '%zd' expects argument of type 'signed size_t', but argument 6 has type 'size_t {aka long unsigned int}' [-Werror=format=]
 #define FUNC_NAME "mc_serialize_str()"
                   ^
serialize.c:175:22: note: in expansion of macro 'FUNC_NAME'
                      FUNC_NAME
                      ^~~~~~~~~
serialize.c: In function 'mc_deserialize_config':
serialize.c:267:19: error: format '%zd' expects argument of type 'signed size_t', but argument 3 has type 'size_t {aka long unsigned int}' [-Werror=format=]
 #define FUNC_NAME "mc_deserialize_config()"
                   ^
serialize.c:269:35: note: in expansion of macro 'FUNC_NAME'
     prepend_error_message (error, FUNC_NAME " at %zd", current_position + 1); \
                                   ^~~~~~~~~
serialize.c:301:17: note: in expansion of macro 'prepend_error_and_exit'
                 prepend_error_and_exit ();
                 ^~~~~~~~~~~~~~~~~~~~~~
serialize.c:267:19: error: format '%zd' expects argument of type 'signed size_t', but argument 3 has type 'size_t {aka long unsigned int}' [-Werror=format=]
 #define FUNC_NAME "mc_deserialize_config()"
                   ^
serialize.c:269:35: note: in expansion of macro 'FUNC_NAME'
     prepend_error_message (error, FUNC_NAME " at %zd", current_position + 1); \
                                   ^~~~~~~~~
serialize.c:313:17: note: in expansion of macro 'prepend_error_and_exit'
                 prepend_error_and_exit ();
                 ^~~~~~~~~~~~~~~~~~~~~~
serialize.c:267:19: error: format '%zd' expects argument of type 'signed size_t', but argument 3 has type 'size_t {aka long unsigned int}' [-Werror=format=]
 #define FUNC_NAME "mc_deserialize_config()"
                   ^
serialize.c:269:35: note: in expansion of macro 'FUNC_NAME'
     prepend_error_message (error, FUNC_NAME " at %zd", current_position + 1); \
                                   ^~~~~~~~~
serialize.c:325:17: note: in expansion of macro 'prepend_error_and_exit'
                 prepend_error_and_exit ();
                 ^~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Andreas Mohr <and@gmx.li>
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>