Move some system includes into c.h, and remove duplicates.

src/backend/access/common/heaptuple.c

src/backend/access/common/indextuple.c

src/backend/access/common/printtup.c

src/backend/access/common/tupdesc.c

src/backend/access/gist/gist.c

src/backend/access/gist/gistget.c

src/backend/access/gist/gistscan.c

src/backend/access/hash/hash.c

src/backend/access/hash/hashovfl.c

src/backend/access/hash/hashpage.c

src/backend/access/hash/hashsearch.c

src/backend/access/hash/hashutil.c

src/backend/access/heap/heapam.c

src/backend/access/heap/stats.c

src/backend/access/index/genam.c

src/backend/access/nbtree/nbtcompare.c

src/backend/access/nbtree/nbtinsert.c

src/backend/access/nbtree/nbtpage.c

src/backend/access/nbtree/nbtree.c

src/backend/access/nbtree/nbtsearch.c