Fix CXX invalid cast from void *.

This is an obvious patch to fix the following build error seen with
--enable-build-with-cxx:

../../src/gdb/rs6000-tdep.c: In function ‘rs6000_frame_cache* rs6000_frame_cache(frame_info*, void**)’:
../../src/gdb/rs6000-tdep.c:3242:15: error: invalid conversion from ‘void*’ to ‘rs6000_frame_cache*’ [-fpermissive]
       return (*this_cache);
              ~^~~~~~~~~~~~

gdb/ChangeLog

	* rs6000-tdep.c (rs6000_frame_cache): Explicitly cast return result
	to avoid invalid conversion from void *.