Make all required includes explicit

Information as seen by IWYU 0.12 + clang 9 on Linux
diff --git a/DateMeter.c b/DateMeter.c
index 6ea108e..c1a097f 100644
--- a/DateMeter.c
+++ b/DateMeter.c
@@ -5,12 +5,15 @@
 in the source distribution for its full text.
 */
 
+#include "config.h" // IWYU pragma: keep
+
 #include "DateMeter.h"
 
-#include "CRT.h"
-
 #include <time.h>
 
+#include "CRT.h"
+#include "Object.h"
+
 
 static const int DateMeter_attributes[] = {
    DATE