libcxx: fix OpenBSD build failure due to missing __get_locale_encoding()

https://github.com/llvm/llvm-project/pull/228306

lib/libcxx/include/__locale_dir/support/openbsd.h

@@ -24,6 +24,9 @@
 #  include <wchar.h>
 #  include <wctype.h>
 #endif
+#if defined(_LIBCPP_BUILDING_LIBRARY)
+#  include <langinfo.h>
+#endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -74,6 +77,10 @@ inline __lconv_t* __localeconv(__locale_t& __loc) {
   __locale_guard __current(__loc);
   return ::localeconv();
 }
+
+inline _LIBCPP_HIDE_FROM_ABI const char* __get_locale_encoding(__locale_t __loc) {
+    return ::nl_langinfo_l(CODESET, __loc);
+}
 #endif // _LIBCPP_BUILDING_LIBRARY
 
 //