LibC: Move SIG_* definitions for sigprocmask into bits/sighow.h

Userland/Libraries/LibC/bits/sighow.h

@@ -0,0 +1,11 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#define SIG_BLOCK 0
+#define SIG_UNBLOCK 1
+#define SIG_SETMASK 2

Userland/Libraries/LibC/signal.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <Kernel/API/POSIX/signal.h>
+#include <bits/sighow.h>
 #include <signal_numbers.h>
 #include <sys/types.h>