Fix Readline 8.1 build on mingw

The mingw build fails with Readline 8.1, because sigprocmask is called
unconditionally.  This patch adds the missing check for
HAVE_POSIX_SIGNALS.

I reported this upstream here:

    https://lists.gnu.org/archive/html/bug-readline/2021-01/msg00011.html

2021-02-06  Tom Tromey  <tom@tromey.com>

	* readline/signals.c (_rl_handle_signal): Add missing check for
	HAVE_POSIX_SIGNALS.