sigframe: cast the pointer to the field ucontext::uc_sigmask to k_rtsigset_t

The type of the field ucontext::uc_sigmask isn't k_rtsigset_t
if the struct ucontext is imported from system headers
rather than provided by an architecture-specific header.
Signed-off-by: Alexander Kartashov <alekskartashov@parallels.com>
Cc: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

sigframe.c

@@ -10,7 +10,7 @@ int construct_sigframe(struct rt_sigframe *sigframe,
 				     struct rt_sigframe *rsigframe,
 				     CoreEntry *core)
 {
-	k_rtsigset_t *blk_sigset = &RT_SIGFRAME_UC(sigframe).uc_sigmask;
+	k_rtsigset_t *blk_sigset = (k_rtsigset_t*)&RT_SIGFRAME_UC(sigframe).uc_sigmask;
 
 	if (core->tc)
 		memcpy(blk_sigset, &core->tc->blk_sigset, sizeof(k_rtsigset_t));