Fix compile errors from 669ba885ceecad83562dc2084057232f7c7fca0f

Sorry, you can't avoid including unit.hpp in all cases.

But, it's still a lot less than it used to be.

src/units/make.cpp

@@ -12,8 +12,7 @@
    See the COPYING file for more details.
 */
 
-#include "units/ptr.hpp"
-#include "units/unit.hpp"
+#include "units/make.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

@@ -13,6 +13,9 @@
 */
 
 #include "units/race.hpp"
+#include "units/ptr.hpp"
+#include "units/unit.hpp"
+
 class config;
 class unit_type;
 class vconfig;