pgsql: Clean up c.h / postgres.h after Assert() move

commit 381d4b70a9854a7b5b9f12d828a0824f8564f1e7 introduced some
compiler warnings:

assert.c:26: warning: no previous prototype for 'ExceptionalCondition'
elog.c: In function 'pg_re_throw':
elog.c:1628: warning: implicit declaration of function 'ExceptionalCondition'
elog.c:1630: warning: 'noreturn' function does return