app_voicemail.c: Fix compile error when IMAP enabled.

Change-Id: I2703f15b4099b4210c68eccf293105d1975c1fc1

apps/app_voicemail.c

@@ -2730,7 +2730,7 @@ static int imap_store_file(const char *dir, const char *mailboxuser, const char
 	}
 	if (fread(buf, 1, len, p) != len) {
 		if (ferror(p)) {
-			ast_log(LOG_ERROR, "Error while reading mail file: %s\n");
+			ast_log(LOG_ERROR, "Error while reading mail file: %s\n", tmp);
 			return -1;
 		}
 	}