cwchar:Use CONFIG_LIBC_WCHAR to only export the wc/mb functions

When a build does not want to use wide or multibyte char
   CONFIG_LIBC_WCHAR is not set. Therefore we should to only
   export the wc/mb functions when defined.

   Regardless of the stat of CONFIG_LIBC_WCHAR the non mb/wc
   definitions such as mbstate_t, wint_t, wctype_t need to be
   exported.

include/cxx/cwchar

@@ -54,6 +54,7 @@ namespace std
   using ::wctype_t;
   using ::size_t;
 
+#ifdef CONFIG_LIBC_WCHAR
   using ::btowc;
   using ::fwprintf;
   using ::fwscanf;
@@ -132,6 +133,7 @@ namespace std
   using ::wmemset;
   using ::wprintf;
   using ::wscanf;
+#endif
 };
 
 #endif // __INCLUDE_CXX_CWCHAR