cleanup: remove system includes from dive.c

Let the various source files include the system headers they need
themselves.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/dive.h

@@ -4,19 +4,13 @@
 
 // dive and dive computer related structures and helpers
 
-#include <stdlib.h>
-#include <stdint.h>
-#include <stdbool.h>
-#include <time.h>
-#include <math.h>
-#include <string.h>
-#include <stdio.h>
-
 #include "divemode.h"
 #include "divecomputer.h"
 #include "equipment.h"
 #include "picture.h"
 
+#include <stdio.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif