c: Finish the phogen module

This commit implements the `phogen_encode` function, which takes a bhash
as argument and generates a phonetic string.

c/phogen/inc/phogen.h

@@ -1,4 +1,10 @@
 #ifndef PHOGEN_H_INCLUDED
 #define PHOGEN_H_INCLUDED
 
+#include <stdbool.h>
+
+#include "bhash.h"
+
+bool phogen_encode(char *out, size_t out_len, bhash_t *bh_in);
+
 #endif /* PHOGEN_H_INCLUDED */