Mark the dc_datetime_mktime parameter as const.

Unlike the corresponding mktime C library function, the libdivecomputer
function will never modify its argument. By marking the parameter as
const this is made more explicit.

include/libdivecomputer/datetime.h

@@ -53,7 +53,7 @@ dc_datetime_gmtime (dc_datetime_t *result,
                     dc_ticks_t ticks);
 
 dc_ticks_t
-dc_datetime_mktime (dc_datetime_t *dt);
+dc_datetime_mktime (const dc_datetime_t *dt);
 
 #ifdef __cplusplus
 }

src/datetime.c

@@ -112,7 +112,7 @@ dc_datetime_gmtime (dc_datetime_t *result,
 }
 
 dc_ticks_t
-dc_datetime_mktime (dc_datetime_t *dt)
+dc_datetime_mktime (const dc_datetime_t *dt)
 {
 	if (dt == NULL)
 		return -1;