meson: Fix failure to detect bsd_auth.h presence

meson: Fix failure to detect bsd_auth.h presence

bsd_auth.h file needs to be included after 'sys/types.h', as documented in
https://man.openbsd.org/authenticate.3

The reason a similar looking stanza works for autoconf is that autoconf
automatically adds AC_INCLUDES_DEFAULT, which in turn includes sys/types.h.

Backpatch to all versions with meson support.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/637haqqyhg2wlz7q6wq25m2qupe67g7f2uupngzui64zypy4x2@ysr2xnmynmu4
Backpatch-through: 16