Unexport yline

It's a method of entityclass, even though it doesn't use anything on the
class and it's not used outside of Entity.cpp.

desktop_version/src/Entity.cpp

@@ -4211,7 +4211,7 @@ float entityclass::hplatformat(const int px, const int py)
     return -1000;
 }
 
-int entityclass::yline( int a, int b )
+static int yline( int a, int b )
 {
     if (a < b) return -1;
     return 1;