Quick fix, future releases will remove libfmt usage.

https://bugs.gentoo.org/986002
https://github.com/diasurgical/DevilutionX/commit/ac2d7fc731bc3
--- a/Source/qol/xpbar.cpp
+++ b/Source/qol/xpbar.cpp
@@ -10,4 +10,5 @@
 
 #include <fmt/core.h>
+#include <fmt/format.h>
 
 #include "control.h"