Fix implicit function declaration strcasecmp

icons-hash.c uses strcasecmp from strings.h, but does not include it.
At least when building against musl with gcc 15, this is a fatal error.

src/icons-hash.c

@@ -23,6 +23,7 @@
 
 #include <stdio.h>
 #include <string.h>
+#include <strings.h>
 #include <stdlib.h>
 #include <limits.h>
 #include "icons.h"