LibC: Make wchar size definitions available from stdint.h

POSIX describes WCHAR_MIN and WCHAR_MAX in stdint.h(0P), while
wchar.h(0P) only says "as described in stdint.h".

As there isn't a trivial path of "may make visible", just move it to a
shared header and include it from both files.

Userland/Libraries/LibC/bits/stdint.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <bits/wchar_size.h>
 #include <sys/cdefs.h>
 
 __BEGIN_DECLS

Userland/Libraries/LibC/bits/wchar_size.h

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2022, Tim Schumacher <timschumi@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#define WCHAR_MAX __WCHAR_MAX__
+#ifdef __WCHAR_MIN__
+#    define WCHAR_MIN __WCHAR_MIN__
+#else
+// Note: This assumes that wchar_t is a signed type.
+#    define WCHAR_MIN (-WCHAR_MAX - 1)
+#endif

Userland/Libraries/LibC/wchar.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <bits/FILE.h>
+#include <bits/wchar_size.h>
 #include <stdarg.h>
 #include <stddef.h>
 #include <sys/cdefs.h>
@@ -17,16 +18,6 @@ __BEGIN_DECLS
 #    define WEOF (0xffffffffu)
 #endif
 
-#undef WCHAR_MAX
-#undef WCHAR_MIN
-#define WCHAR_MAX __WCHAR_MAX__
-#ifdef __WCHAR_MIN__
-#    define WCHAR_MIN __WCHAR_MIN__
-#else
-// Note: This assumes that wchar_t is a signed type.
-#    define WCHAR_MIN (-WCHAR_MAX - 1)
-#endif
-
 typedef __WINT_TYPE__ wint_t;
 typedef unsigned long int wctype_t;