coerce tests/overmap_test.cpp to compile despite include shuffling.

this might warrant pushing json.h into the includes in common_types.h but i don't fully understand the consequences of this.

tests/overmap_test.cpp

@@ -1,6 +1,7 @@
 #include <memory>
 #include <vector>
 
+#include "json.h"
 #include "all_enum_values.h"
 #include "ammo.h"
 #include "calendar.h"