keg: Install gziped info files during linking

[ywwry66]

Currently, brew link installs *.info files to #{HOMEBREW_PREFIX}/share/info/dir using the install_info method. However, some formulae (e.g., Emacs) also ship *.info.gz files, which are only symlinked but not installed.

This PR allows *.info.gz files to be installed during linking in addition to the *.info files.

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[MikeMcQuaid]

Thanks again @ywwry66!