Support OpenBSD tar by using explicit tar -xzf

[sirn]

OpenBSD tar won't try to extract the compressed archive if no format type (z/j/...) is specified and trying to install pg_query gem on OpenBSD system will result in the following error:

tar: input compressed with gzip; use the -z option to decompress it
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog
        --without-make-prog
        --srcdir=.
        --curdir
        --ruby=/home/sirn/.asdf/installs/ruby/2.6.2/bin/$(RUBY_BASE_NAME)
extconf.rb:22:in `<main>': ERROR (RuntimeError)

extconf failed, exit code 1

This PR fixes the error by adding z to the tar command in extconf.

[lfittl]

@sirn Thanks for the contribution!