Partially revert ca51442277

Taking an argument, especially by reference, does not require a full type.
Including unit.hpp in make.hpp utterly defeats the point of make.hpp's existence.

src/units/make.cpp

@@ -13,6 +13,7 @@
 */
 
 #include "units/make.hpp"
+#include "units/unit.hpp"
 
 unit_ptr make_unit_ptr(const config& cfg, bool use_traits, const vconfig* vcfg){
     return { new unit(cfg, use_traits, vcfg) };

src/units/make.hpp

@@ -14,7 +14,6 @@
 
 #include "units/race.hpp"
 #include "units/ptr.hpp"
-#include "units/unit.hpp"
 
 class config;
 class unit_type;