BUG/MINOR: ssl: Stop passing dynamic strings as format arguments

gcc complains rightfully:

src/ssl_sock.c: In function ‘ssl_load_global_issuers_from_path’:
src/ssl_sock.c:9860:4: warning: format not a string literal and no format arguments [-Wformat-security]
    ha_warning(warn);
    ^

Introduced in 70df7bf19cebd5593c0abb01923e6c9f72961da6.
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index e30bb8a..ade5ffc 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -9857,7 +9857,7 @@
 			goto next;
 		ssl_load_global_issuer_from_BIO(in, fp, &warn);
 		if (warn) {
-			ha_warning(warn);
+			ha_warning("%s", warn);
 			free(warn);
 			warn = NULL;
 		}