player: Add missing header

As required by rand(3).

src/player/src/human_player_gui.c

@@ -16,6 +16,7 @@
 #include <stddef.h>
 #include <stdio.h>
 #include <stdint.h>
+#include <stdlib.h>
 
 enum {X_OFF = 8, Y_OFF = 8};
 

src/player/src/player.c

@@ -3,6 +3,7 @@
 #include <unit.h>
 #include <terrain.h>
 #include <stddef.h>
+#include <stdlib.h>
 
 void player_update(struct player *const p)
 {