Revert "fcntl.h: fix a missing `struct timespec` definition"

This reverts commit 657c4a9d6c2e649d86307f46ce9d5e44192f6e7a.

This breaks a lot of package cross-compilation as lvm, ruby,
ipsec-tools. :(

include/fcntl.h

@@ -34,9 +34,6 @@ __BEGIN_DECLS
 
 /* For XPG all symbols from <sys/stat.h> should also be available.  */
 #ifdef __USE_XOPEN
-# ifdef __USE_XOPEN2K8
-#  define __need_timespec
-# endif
 # include <sys/stat.h>
 #endif