posix: signal: define max rt signals as zero when not enabled

When CONFIG_POSIX_SIGNAL=n, default CONFIG_POSIX_RTSIG_MAX to
0 so that the `sigset_t` type can be defined.

Also define RTSIG_MAX.

This allows native_sim and other platforms to build without
error.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>