pgsql: Use .h for the file containing the page checksum code fragment

Use .h for the file containing the page checksum code fragment

Commit 5e13b0f24 used a .c file for a file containing a code fragment,
to avoid adding an exception to headerscheck. That turned out to be
too clever, since it meant installation didn't happen by the usual
mechanism. Make it look like a normal header and add the requisite
exception.

Bug: #19450
Reported-by: RekGRpth <rekgrpth(at)gmail(dot)com>
Discussion: https://postgr.es/m/19450-bb0612c50c6786e5@postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/8c3e22a8f8b9af94b519b6e223db0912d552093e

Modified Files
--------------
src/backend/storage/page/checksum.c | 4 ++--
.../storage/{checksum_block.inc.c => checksum_block_internal.h} | 6 +++---
src/include/storage/checksum_impl.h | 2 +-
src/tools/pginclude/headerscheck | 4 +++-
4 files changed, 9 insertions(+), 7 deletions(-)