Clang says: 'osl_detail_ObjectRegistry_getMutex' has C-linkage
specified, but returns user-defined type '::osl::Mutex &' which is
incompatible with C.

Let's continue disabling that warning also also in 4.0 and later.

Change-Id: I892bdc7e7aa6d546adf6b1c388e4c149db738948