LibC: Add a missing mbstowcs() declaration

This ensures it gets a C linkage, and so fixes the binutils build.

Libraries/LibC/stdlib.h

@@ -43,6 +43,7 @@ int system(const char* command);
 char* mktemp(char*);
 char* mkdtemp(char*);
 void* bsearch(const void* key, const void* base, size_t nmemb, size_t size, int (*compar)(const void*, const void*));
+size_t mbstowcs(wchar_t*, const char*, size_t);
 
 #define RAND_MAX 32767
 int rand();