Add missing newline in one libpq error message.

Oversight in commit a59c79564.  Back-patch, as that was.
Noted by Peter Eisentraut.

Discussion: https://postgr.es/m/7f85ef6d-250b-f5ec-9867-89f0b16d019f@enterprisedb.com

src/interfaces/libpq/fe-secure-openssl.c

@@ -1234,7 +1234,7 @@ initialize_SSL(PGconn *conn)
 		if (!S_ISREG(buf.st_mode))
 		{
 			appendPQExpBuffer(&conn->errorMessage,
-							  libpq_gettext("private key file \"%s\" is not a regular file"),
+							  libpq_gettext("private key file \"%s\" is not a regular file\n"),
 							  fnbuf);
 			return -1;
 		}