Silence compiler warning, hopefully.

Absorb commit e5e04c962a5d12eebbf867ca25905b3ccc34cbe0 from upstream
IANA code, in hopes of silencing warnings from MSVC about negating
a bool value.

Discussion: https://postgr.es/m/20190719035347.GJ1859@paquier.xyz

src/timezone/zic.c

@@ -2398,7 +2398,7 @@ writezone(const char *const name, const char *const string, char version,
 		if (pass == 1 && !want_bloat())
 		{
 			utcnt = stdcnt = thisleapcnt = 0;
-			thistimecnt = -locut - hicut;
+			thistimecnt = -(locut + hicut);
 			thistypecnt = thischarcnt = 1;
 			thistimelim = thistimei;
 		}