Fix portability problem induced by commit a6f6b781.

pg_xlogdump includes bufmgr.h.  With a compiler that emits code for
static inline functions even when they're unreferenced, that leads
to unresolved external references in the new static-inline version
of BufferGetPage().  So hide it with #ifndef FRONTEND, as we've done
for similar issues elsewhere.  Per buildfarm member pademelon.