caca_internals: export _caca_alloc2d

It is used by the image viewer:

/usr/bin/ld: img2txt-common-image.o: in function `load_image':
../../src/common-image.c:164: undefined reference to `_caca_alloc2d'

Fixes #59.

caca/caca_internals.h

@@ -268,7 +268,7 @@ extern int _caca_pop_event(caca_display_t *, caca_privevent_t *);
 extern void _caca_set_term_title(char const *);
 
 /* Internal memory function */
-extern void *_caca_alloc2d(size_t width, size_t height, size_t elem_size);
+__extern void *_caca_alloc2d(size_t width, size_t height, size_t elem_size);
 
 /* Profiling functions */
 #if defined PROF