Define DC_TIMEZONE_NONE as a signed integer

The hexadecimal value 0x80000000 is too large to be represented as a
signed 32bit integer. Therefore the default type for the constant is an
unsigned 32bit integer. This is a bit annoying because the timezone
field is actually defined as a signed integer, and thus comparisions
produce -Wsign-compare compiler warnings.

Fixed by switching to INT_MIN, which is the same underlying value but
interpreted as a signed integer.

include/libdivecomputer/datetime.h

@@ -22,11 +22,13 @@
 #ifndef DC_DATETIME_H
 #define DC_DATETIME_H
 
+#include <limits.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif /* __cplusplus */
 
-#define DC_TIMEZONE_NONE 0x80000000
+#define DC_TIMEZONE_NONE INT_MIN
 
 #if defined (_WIN32) && !defined (__GNUC__)
 typedef __int64 dc_ticks_t;