Define ENEMY_NUM in bossrush.cpp

ENEMY_NUM is not used in bossrush mode. However, I had to define it in bossrush.cpp, since it is decleared in the file definitions.hpp and the linker will look for it.
Another solution could be defining ENEMY_NUM in another .cpp file (e.g.
functions.cpp), but it wouldn't change that much.

bossrush.cpp

@@ -23,6 +23,8 @@ using std::endl;
 using std::min;
 using std::max;
 
+int ENEMY_NUM=1;	// This variable isn't used, but we have to define it since it is decleared in definitions.hpp"
+
 int main(int argc,char** argv)
 {