common-image: avoid implicit function declaration

common-image.c: In function ‘load_image’:
common-image.c:164:18: warning: implicit declaration of function ‘_caca_alloc2d’ [-Wimplicit-function-declaration]
  164 |     im->pixels = _caca_alloc2d(im->w, im->h, depth);
      |                  ^~~~~~~~~~~~~

src/common-image.c

@@ -22,6 +22,7 @@
 #endif
 
 #include "caca.h"
+#include "caca_internals.h"
 
 #include "common-image.h"