use unsigned bitmask for consistency in ksigaction

the type doesn't actually matter, just the size, but it's nice to be
consistent...

src/internal/ksigaction.h

@@ -2,5 +2,5 @@ struct k_sigaction {
 	void (*handler)(int);
 	unsigned long flags;
 	void (*restorer)(void);
-	long mask[2];
+	unsigned long mask[2];
 };