[Darwin] Add include guard to darwin-protos.h

The Darwin protos header is missing an include guard, this adds one.

gcc/ChangeLog:

2019-11-08  Iain Sandoe  <iain@sandoe.co.uk>

	* config/darwin-protos.h: Add include quard.

From-SVN: r277993

gcc/ChangeLog

+2019-11-08  Iain Sandoe  <iain@sandoe.co.uk>
+
+	* config/darwin-protos.h: Add include quard.
+
 2019-11-08  Andrew MacLeod  <amacleod@redhat.com>
 
 	* range-op.h (range_operator::fold_range): Return result in a

gcc/config/darwin-protos.h

@@ -17,6 +17,9 @@ You should have received a copy of the GNU General Public License
 along with GCC; see the file COPYING3.  If not see
 <http://www.gnu.org/licenses/>.  */
 
+#ifndef CONFIG_DARWIN_PROTOS_H
+#define CONFIG_DARWIN_PROTOS_H
+
 extern void darwin_init_sections (void);
 extern int name_needs_quotes (const char *);
 
@@ -123,3 +126,5 @@ extern void darwin_override_options (void);
 extern void darwin_patch_builtins (void);
 extern void darwin_rename_builtins (void);
 extern bool darwin_libc_has_function (enum function_class fn_class);
+
+#endif /* CONFIG_DARWIN_PROTOS_H */