fs: Fix RomFS building when zero byte files are present

When zero byte files are present, the key (offset) for that file is identical to the file right after. A std::map isn't able to fit key-value pairs with identical keys (offsets), therefore, the solution is to use std::multimap which permits multiple entries with the same key.

This most prominently fixes Pokemon Sword and Shield weather with any RomFS mod applied.

src/core/file_sys/fsmitm_romfsbuild.h

@@ -43,7 +43,7 @@ public:
     ~RomFSBuildContext();
 
     // This finalizes the context.
-    std::map<u64, VirtualFile> Build();
+    std::multimap<u64, VirtualFile> Build();
 
 private:
     VirtualDir base;