Fix header inclusion order in c.h.

Fix header inclusion order in c.h.

Commit 962da900a added #include <stdint.h> to postgres_ext.h, which
broke c.h's header ordering rule.

The system headers on some systems would then lock down off_t's size in
private macros, before they'd had a chance to see our definition of
_FILE_OFFSET_BITS (and presumably other things).  This was picked up by
perl's ABI compatibility checks on some 32 bit systems in the build
farm.

Move #include "postgres_ext.h" down below the system header section, and
make the comments clearer (thanks to Tom for the new wording).

Diagnosed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/2397643.1733347237%40sss.pgh.pa.us