gcc is picky, fix gcc warnings

src/kernel/plane.h

@@ -22,6 +22,7 @@ OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 extern "C" {
 #endif
 
+    struct faction;
     struct region;
     struct faction;
     struct plane;

src/kernel/region.h

@@ -66,6 +66,7 @@ extern "C" {
     struct message_list;
     struct rawmaterial;
     struct item;
+    struct faction;
 
 #define MORALE_TAX_FACTOR 0.005 /* 0.5% tax per point of morale */
 #define MORALE_MAX 10           /* Maximum morale allowed */