libcacard: Fix compilation for older versions of glib (bug #1258168)

libcacard: Fix compilation for older versions of glib (bug #1258168)

See https://bugs.launchpad.net/bugs/1258168

libcacard/vscclient.c: In function 'do_socket_read':
libcacard/vscclient.c:410: warning: implicit declaration of function 'g_warn_if_reached'
libcacard/vscclient.c:410: warning: nested extern declaration of 'g_warn_if_reached'
libcacard/vscclient.c: In function 'main':
libcacard/vscclient.c:763: warning: implicit declaration of function 'g_byte_array_unref'
libcacard/vscclient.c:763: warning: nested extern declaration of 'g_byte_array_unref'
...
libcacard/vscclient.o: In function `do_socket_read':
libcacard/vscclient.c:410: undefined reference to `g_warn_if_reached'
libcacard/vscclient.o: In function `main':
libcacard/vscclient.c:763: undefined reference to `g_byte_array_unref'

g_warn_if_reached was added in glib 2.16, and g_byte_array_unref is
supported since glib 2.22. QEMU requires glib 2.12, so both names must
not be used.

Instead of showing a warning for code which should not be reached,
vscclient better stop running, so g_warn_if_reached is not useful for
vscclient.

In libcacard/vsclient.c, g_byte_array_unref can be replaced by
g_byte_array_free. This is not generally true, so adding a compatibility
layer in include/glib-compat.h is no option here.

Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Reported-by: Don Slutz <dslutz@verizon.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>