Make another .h file accessible from C++.

src/core/lib/security/context/security_context.h

@@ -37,6 +37,10 @@
 #include "src/core/lib/iomgr/pollset.h"
 #include "src/core/lib/security/credentials/credentials.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* --- grpc_auth_context ---
 
    High level authentication context object. Can optionally be chained. */
@@ -111,4 +115,8 @@ grpc_auth_context *grpc_auth_context_from_arg(const grpc_arg *arg);
 grpc_auth_context *grpc_find_auth_context_in_args(
     const grpc_channel_args *args);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* GRPC_CORE_LIB_SECURITY_CONTEXT_SECURITY_CONTEXT_H */