xdiff: drop system includes in xutils.c

xdiff: drop system includes in xutils.c

After b46054b374 ("xdiff: use git-compat-util", 2019-04-11), two system
headers added with 6942efcfa9 ("xdiff: load full words in the inner loop
of xdl_hash_record", 2012-04-06) to xutils.c are no longer needed and
could conflict as shown below from an OpenIndiana build:

In file included from xdiff/xinclude.h:26:0,
                 from xdiff/xutils.c:25:
./git-compat-util.h:4:0: warning: "_FILE_OFFSET_BITS" redefined
 #define _FILE_OFFSET_BITS 64

In file included from /usr/include/limits.h:37:0,
                 from xdiff/xutils.c:23:
/usr/include/sys/feature_tests.h:231:0: note: this is the location of the previous definition
 #define _FILE_OFFSET_BITS 32

Make sure git-compat-util.h is the first header (through xinclude.h)

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>