cext: fix a warning about differing const qualifiers on Windows

The prototype takes a 'void *', but self->offsets is a 'const char **'.

    mercurial/cext/revlog.c(2011) : warning C4090: 'function' : different
            'const' qualifiers

mercurial/cext/revlog.c

@@ -2005,7 +2005,7 static void _index_clearcaches(indexObje
 		self->cache = NULL;
 	}
 	if (self->offsets) {
-		PyMem_Free(self->offsets);
+		PyMem_Free((void *)self->offsets);
 		self->offsets = NULL;
 	}
 	if (self->nt != NULL) {