unistd: fix #if condition

Commit dfa593d4d881116723a4401b466ea964fb12327b ("syncfs: add system
call support") modified the #if condition around the definition of the
syncfs() prototype in a way that doesn't build, causing build failures
of any file including <unistd.h>:

output/host/usr/powerpc-buildroot-linux-uclibc/sysroot/usr/include/unistd.h:988:14: error: #if with no expression
 #if __USE_GNU

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reported-by: Xi-Sheng Luo <lxsjason@gmail.com>

include/unistd.h

@@ -1073,7 +1073,7 @@ extern char *getpass (const char *__prompt) __nonnull ((1));
 extern int fsync (int __fd);
 #endif /* Use BSD || X/Open || Unix98.  */
 
-#if __USE_GNU
+#if defined __USE_GNU
 /* Make all changes done to all files on the file system associated
  *    with FD actually appear on disk.  */
 extern int syncfs (int __fd) __THROW;