tests: fix build with fresh glibc

Workaround additional incompatibility between <sys/stat.h> and
<asm/stat.h> introduced by glibc-2.27.9000-566-gfd70af4.

* tests/xstatx.c (statx, statx_timestamp): Redefine before the inclusion
of <sys/stat.h> to avoid conflicts between definitions provided by the
linux kernel headers and the GNU libc.

tests/xstatx.c

@@ -64,10 +64,14 @@ typedef off_t libc_off_t;
 
 # define stat libc_stat
 # define stat64 libc_stat64
+# define statx libc_statx
+# define statx_timestamp libc_statx_timestamp
 # include <fcntl.h>
 # include <sys/stat.h>
-# undef stat
+# undef statx_timestamp
+# undef statx
 # undef stat64
+# undef stat
 
 # undef st_atime
 # undef st_mtime