LibC, LibM: Add functions needed to compile python3

Author: https://github.com/lnzero1dev
Commit: e7affa24dc
Pull-request: https://github.com/SerenityOS/serenity/pull/763
Reviewed-by: https://github.com/awesomekling ✅
Reviewed-by: https://github.com/bugaevc

Libraries/LibC/stdlib.h

@@ -46,6 +46,7 @@ void* bsearch(const void* key, const void* base, size_t nmemb, size_t size, int
 size_t mbstowcs(wchar_t*, const char*, size_t);
 size_t mbtowc(wchar_t*, const char*, size_t);
 int wctomb(char*, wchar_t);
+size_t wcstombs(char*, const wchar_t*, size_t);
 
 #define RAND_MAX 32767
 int rand();