watchdog: add includes for geteuid

05a3e2bad9 added `geteuid` use but didn't
add the needed headers. This works by chance on libstdc++ systems via
transitive includes, but it doesn't work on libc++ systems, as reported
downstream in Gentoo at https://bugs.gentoo.org/933120.

src/watchdog.cpp

@@ -8,6 +8,8 @@
 */
 
 #include "watchdoglogging.h"
+#include <unistd.h>
+#include <sys/types.h>
 #include <QCoreApplication>
 #include <QTimer>
 #include <systemd/sd-daemon.h>