Fix building with clang 16

Fix building with clang 16

From Clang 16 (and GCC 13) -Wimplicit-function-declaration has been
turned on by default. This resutls in warnings such as

song_hash.c: In function ‘insert_song’:
song_hash.c:22:39: warning: implicit declaration of function ‘malloc’ [-Wimplicit-function-declaration]
   22 |         new_elt = (struct hash_elt *) malloc(sizeof(struct hash_elt));
      |                                       ^~~~~~

This patch should fix these warnings. The warnings can be reproceded by
both recent version of Clang and GCC.

First discovered on Gentoo Linux: https://bugs.gentoo.org/882321

Signed-off-by: Brahmajit Das <brahmajit.xyz@gmail.com>