Properly include coalesce.h in custom_free when needed.

src/custom_free.c

@@ -4,6 +4,9 @@
 #ifdef HAVE_LOCKS
 #include "posix_lock.h"
 #endif /* HAVE_LOCKS */
+#ifdef WITH_COALESCING
+#include "coalesce.h"
+#endif /* WITH_COALESCING */
 #include "block_header.h"
 
 void custom_ahfree(allocator_t *allocator, heap_t* heap, void *ptr) {