gcc-11 needs const operator()

https://bugs.gentoo.org/768204
--- a/BPGame.h
+++ b/BPGame.h
@@ -53,7 +53,7 @@ class BPMiniGame;
 class SpriteFont;
 
 struct cmp_str {
-	bool operator()(char const *a, char const *b) {
+	bool operator()(char const *a, char const *b) const {
 		return std::strcmp(a, b) < 0;
 	}
 };