fstests: use '-std=gnu11' to fix build errors with gcc-15

fstests: use '-std=gnu11' to fix build errors with gcc-15

GCC-15 does a big change, it changes the default C compilation
language standard to -std=gnu23. That cause lots of "old style"
C codes hit building errors. On the other word, current xfstests
can't be used with GCC-15. So use a fixed language standard --
"-std=gnu11" (to match the kernel and xfsprogs).

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>