Add #include guards around bridging header

Author: DougGregor

test/ClangImporter/Inputs/c-bridging-header.h

@@ -1,6 +1,11 @@
+#ifndef C_BRIDGING_HEADER_H
+#define C_BRIDGING_HEADER_H
+
 #include "ctypes.h"
 #include "macros.h"
 
 typedef struct {
   double x, y;
 } MyPoint;
+
+#endif