This allows removing #include "llerror.h" from llpointer.h.
Also remove #include "llmutex.h" as a heavy way to get
<boost/functional/hash.hpp>.
That requires adding #include "llmutex.h" to llimage.h, llnotifications.h,
llwatchdog.cpp and llvolumemgr.cpp, which were inheriting it from llpointer.h.
