glapi: Enclose glapi.h in an extern "C" block when included by C++.

Fixes a build failure introduced in commit
b7fa0d0727.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-and-tested-by: Paul Berry <stereotype441@gmail.com>

src/mapi/glapi/glapi.h

@@ -44,6 +44,9 @@
 #ifndef _GLAPI_H
 #define _GLAPI_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 #ifdef _GLAPI_NO_EXPORTS
 #  define _GLAPI_EXPORT
@@ -177,5 +180,8 @@ _glapi_noop_enable_warnings(unsigned char enable);
 _GLAPI_EXPORT void
 _glapi_set_warning_func(_glapi_proc func);
 
+#ifdef __cplusplus
+}
+#endif
 
 #endif /* _GLAPI_H */