LibC: Add uchar.h

This was added in POSIX issue 8 and some ports end up with very weird
compile errors if this does not exist :D

userspace/libraries/LibC/include/uchar.h

@@ -0,0 +1,21 @@
+#ifndef _UCHAR_H
+#define _UCHAR_H 1
+
+// https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/uchar.h.html
+
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+#include <stddef.h>
+#include <stdint.h>
+#include <wchar.h>
+
+size_t c16rtomb(char* __restrict__ s, char16_t c16, mbstate_t* __restrict__ ps);
+size_t c32rtomb(char* __restrict__ s, char32_t c32, mbstate_t* __restrict__ ps);
+size_t mbrtoc16(char16_t* __restrict__ pc16, const char* __restrict__ s, size_t n, mbstate_t* __restrict__ ps);
+size_t mbrtoc32(char32_t* __restrict__ pc32, const char* __restrict__ s, size_t n, mbstate_t* __restrict__ ps);
+
+__END_DECLS
+
+#endif