time: add a tm_gmtoff field to C.tm (a BSD and GNU extension) (#20907)

vlib/time/time_nix.c.v

@@ -17,6 +17,7 @@ pub struct C.tm {
 	tm_wday   int
 	tm_yday   int
 	tm_isdst  int
+	tm_gmtoff int
 }
 
 fn C.timegm(&C.tm) C.time_t

vlib/time/time_test.v

@@ -344,3 +344,16 @@ fn test_parse_three_letters_month() {
 	tm_tm := time.parse_format(tm_s, format)!
 	assert tm_tm.month == tm.month
 }
+
+fn test_tm_gmtoff() {
+	$if windows {
+		return
+	} $else {
+		rawtime := i64(0) // C.time_t{}
+
+		C.time(&rawtime) // C.tm{}
+
+		info := C.localtime(&rawtime)
+		assert info.tm_gmtoff == time.now().unix - time.utc().unix
+	}
+}