Se https://gitlab.alpinelinux.org/alpine/aports/-/issues/16106

diff -upr ssmtp-2.64.orig/ssmtp.c ssmtp-2.64/ssmtp.c
--- ssmtp-2.64.orig/ssmtp.c	2024-05-12 12:51:08.577436203 +0200
+++ ssmtp-2.64/ssmtp.c	2024-05-12 12:51:26.934150173 +0200
@@ -25,6 +25,7 @@
 #include <string.h>
 #include <ctype.h>
 #include <netdb.h>
+#include <libgen.h>
 #ifdef HAVE_SSL
 #include <openssl/crypto.h>
 #include <openssl/x509.h>