Put external declarations into header files.

src/backend/access/gist/gist.c

src/backend/access/hash/hash.c

src/backend/access/heap/heapam.c

src/backend/access/nbtree/nbtree.c

src/backend/access/nbtree/nbtutils.c

src/backend/access/rtree/rtree.c

src/backend/access/transam/rmgr.c

src/backend/access/transam/transam.c

src/backend/access/transam/xact.c

src/backend/access/transam/xlog.c

src/backend/access/transam/xlogutils.c

src/backend/bootstrap/bootparse.y

src/backend/bootstrap/bootstrap.c

src/backend/commands/async.c

src/backend/optimizer/geqo/geqo_misc.c

src/backend/postmaster/postmaster.c

src/backend/storage/ipc/ipc.c

src/backend/storage/ipc/ipci.c

src/backend/storage/ipc/shmem.c

src/backend/storage/smgr/mm.c