games/freeminer: unbreak after r540853

In file included from src/script/common/c_content.cpp:39:
In file included from /usr/local/include/json/json.h:10:
[...]
/usr/local/include/json/value.h:107:6: error: redefinition of 'ValueType'
enum ValueType {

^

src/json/json.h:398:9: note: previous definition is here

enum ValueType
     ^

Reported by: pkg-fallout