Make it possible to use gbm with c++

NOTE: This is a candiate for 7.11

src/gbm/main/gbm.h

@@ -28,6 +28,11 @@
 #ifndef _GBM_H_
 #define _GBM_H_
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
 #define __GBM__ 1
 
 #include <stdint.h>
@@ -97,4 +102,8 @@ gbm_bo_get_handle(struct gbm_bo *bo);
 void
 gbm_bo_destroy(struct gbm_bo *bo);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif