zoneinfo: fix makefile rules script

In make target rules, one needs to use subshell if there are
change directory commands that should only have an effect on the
other commands in the same line. Otherwise, if make is not running in
compatibility mode (for example, when -j flag is specified), commands
would be executed in a single shell and lines following the "cd" might
not work as expected.

Adjust the target script lines that use "cd" to run in a subshell
by adding appropriate parenthesis.

Reviewed by: sjg
Differential Revision: https://reviews.freebsd.org/D42608