Include <limits.h> in fe-auth.c, to get CHAR_BIT reliably.

Include <limits.h> in fe-auth.c, to get CHAR_BIT reliably.

fe-auth.c references CHAR_BIT since commit 3a465cc67, but it
did not #include <limits.h>, which per POSIX is where that
symbol is defined.  This escaped notice so far because
(a) on most platforms, <sys/param.h> pulls in <limits.h>,
(b) even if yours doesn't, OpenSSL pulls it in, so compiling
with --with-openssl masks the omission.

Per bug #18026 from Marcel Hofstetter.  Back-patch to v16.

Discussion: https://postgr.es/m/18026-d5bb69f79cd16203@postgresql.org