sys/unistd.h: define GNU extensions SEEK_DATA and SEEK_HOLE

SEEK_DATA and SEEK_HOLE are GNU nonstandard extensions also present
in Solaris, FreeBSD, and DragonFly BSD; they are proposed for inclusion
in the next POSIX revision (Issue 8).

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

newlib/libc/include/sys/unistd.h

@@ -315,6 +315,10 @@ int	unlinkat (int, const char *, int);
 # define	SEEK_SET	0
 # define	SEEK_CUR	1
 # define	SEEK_END	2
+#if __GNU_VISIBLE
+# define	SEEK_DATA	3
+# define	SEEK_HOLE	4
+#endif
 
 #include <sys/features.h>