pgsql: Make <assert.h> consistently available in frontend and backend

Make <assert.h> consistently available in frontend and backend

Previously, c.h made <assert.h> only available in frontends (#ifdef
FRONTEND), which was probably reasonable, because the only thing it
would give you is assert(), which you generally shouldn't use in the
backend. But with C11, <assert.h> also makes available
static_assert(), which would be useful everywhere. So this patch
moves <assert.h> to the commonly available header files in c.h and
fixes a small complication in regcustom.h that resulted from that.

Co-authored-by: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/CA%2BhUKGKvr0x_oGmQTUkx%3DODgSksT2EtgCA6LmGx_jQFG%3DsDUpg%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/795e94c70cf13f60b2cb4010f29a1bca542d2031

Modified Files
--------------
src/include/c.h | 2 +-
src/include/regex/regcustom.h | 1 +
2 files changed, 2 insertions(+), 1 deletion(-)