LibC: Remove our iconv.h header and add libiconv port

If I ever end up writing my own iconv, i will drop the port :D

userspace/libraries/LibC/include/iconv.h

@@ -1,21 +0,0 @@
-#ifndef _ICONV_H
-#define _ICONV_H 1
-
-// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/iconv.h.html
-
-#include <sys/cdefs.h>
-
-__BEGIN_DECLS
-
-#define __need_size_t
-#include <sys/types.h>
-
-typedef void* iconv_t;
-
-size_t	iconv(iconv_t cd, char** __restrict inbuf, size_t* __restrict inbytesleft, char** __restrict outbuf, size_t* __restrict outbytesleft);
-int		iconv_close(iconv_t cd);
-iconv_t	iconv_open(const char* tocode, const char* fromcode);
-
-__END_DECLS
-
-#endif