Make all required includes explicit

Information as seen by IWYU 0.12 + clang 9 on Linux

ClockMeter.c

@@ -5,12 +5,15 @@ Released under the GNU GPLv2, see the COPYING file
 in the source distribution for its full text.
 */
 
+#include "config.h" // IWYU pragma: keep
+
 #include "ClockMeter.h"
 
-#include "CRT.h"
-
 #include <time.h>
 
+#include "CRT.h"
+#include "Object.h"
+
 
 static const int ClockMeter_attributes[] = {
    CLOCK