parisc: Define sigset_t in parisc uapi header

The util-linux debian package fails to build on parisc, because
sigset_t isn't defined in asm/signal.h when included from userspace.
Move the sigset_t type from internal header to the uapi header to fix the
build.

Link: https://buildd.debian.org/status/fetch.php?pkg=util-linux&arch=hppa&ver=2.40-7&stamp=1714163443&raw=0


Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # v6.0+