omcache.h: add extern "C" for c++ use and drop unused includes
omcache.h: add extern "C" for c++ use and drop unused includes

OMcache no longer uses any vararg functions so stdarg.h isn't needed.
stddef.h is not needed either as all types we use are in sys/types.h.

ohmu/omcache,ohmu/omcache,thedrow/omcache,CrowdStrike/omcache,CrowdStrike/omcache,thedrow/omcache