Flag unreferrenced functions only declared in the main file, not an include

...which appears to be a good heuristic to identify functions that are either
unused or should better be declared just once in an include file.  (It also
filters out SAL_DLLPUBLIC extern "C" function definitions, which are most likely
meant to be referenced dynamically via dlsym.)

Change-Id: I7fb78cb836b971791704851535dcfbda2b2f5bc0