Add cache.h

The functions found in cache.c are only used by cgit.c, so there's no
point in rebuilding all object files when the cache interface is changed.


Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.c

@@ -7,6 +7,7 @@
  */
 
 #include "cgit.h"
+#include "cache.h"
 #include "cmd.h"
 #include "ui-shared.h"