raise: fix mingw handling of SIGPIPE

raise: fix mingw handling of SIGPIPE

When compiling for mingw, I see:

  CC     sigprocmask.lo
sigprocmask.c: In function '_gl_raise_SIGPIPE':
sigprocmask.c:349:1: warning: control reaches end of non-void function

which means that raise(SIGPIPE) would be using an uninitialized
value as its return.

* lib/sigprocmask.c (_gl_raise_SIGPIPE): Provide a return value.

Signed-off-by: Eric Blake <eblake@redhat.com>