Can't completely get rid of #ifndef FRONTEND in palloc.h :-(

Can't completely get rid of #ifndef FRONTEND in palloc.h :-(

pg_controldata includes postgres.h not postgres_fe.h, so utils/palloc.h
must be able to compile in a "#define FRONTEND" context.  It appears that
Solaris Studio is smart enough to persuade us to define PG_USE_INLINE,
but not smart enough to not make a copy of unreferenced static functions;
which leads to an unsatisfied reference to CurrentMemoryContext.  So we
need an #ifndef FRONTEND around that declaration.  Per buildfarm.