tests: fix virfilewrapper

tests: fix virfilewrapper

If __lxstat() and __xstat() functions are not available, build fails with:

  CC       virfilewrapper.o
virfilewrapper.c:180:5: error: no previous prototype for function '__lxstat' [-Werror,-Wmissing-prototypes]
int __lxstat(int ver, const char *path, struct stat *sb)
    ^
virfilewrapper.c:208:5: error: no previous prototype for function '__xstat' [-Werror,-Wmissing-prototypes]
int __xstat(int ver, const char *path, struct stat *sb)

Luckily, we already check presence of these functions in configure
using AC_CHECK_FUNCS, so just don't wrap these if they're not available.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>