cleanup: move declaration of utc_mk* functions to new subsurface-time.h header

No point in slurping in all of dive.h for translation units that only
want to do some time manipulation without ever touching a dive.

Don't call the header "time.h", because we don't want to end up in a
confusion with the system header of the same name.

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

core/subsurface-time.h

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: GPL-2.0
+#ifndef TIME_H
+#define TIME_H
+
+#include "units.h"
+#include <time.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern timestamp_t utc_mktime(struct tm *tm);
+extern void utc_mkdate(timestamp_t, struct tm *tm);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif