locale.c: Silence compiler warning

This as only emitted on Windows VS before 2015.

locale.c

@@ -2003,6 +2003,8 @@ S_new_numeric(pTHX_ const char *newnum, bool force)
                                                              NULL, NULL));
     Safefree(scratch_buffer);
 
+#    else
+    PERL_UNUSED_VAR(C_thousands_sep);
 #    endif
 
     PL_numeric_standard = PL_numeric_underlying_is_standard;