shared-mime-info: Depend on libxml for Linuxbrew
shared-mime-info: Depend on libxml for Linuxbrew

Fixes:
==> Pouring shared-mime-info-1.9.x86_64_linux.bottle.tar.gz
==> /home/linuxbrew/.linuxbrew/Cellar/shared-mime-info/1.9/bin/update-mime-database /home/linuxbrew/.linuxbrew/share/mime
Last 15 lines from /home/linuxbrew/.cache/Homebrew/Logs/shared-mime-info/post_install.01.update-mime-database:
2018-04-20 17:40:03 +0000

/home/linuxbrew/.linuxbrew/Cellar/shared-mime-info/1.9/bin/update-mime-database
/home/linuxbrew/.linuxbrew/share/mime

/home/linuxbrew/.linuxbrew/Cellar/shared-mime-info/1.9/bin/update-mime-database: error while loading shared libraries: libxml2.so.2: cannot open shared object file: No such file or directory
Warning: The post-install step did not complete successfully
You can try again using `brew postinstall shared-mime-info`

Closes Linuxbrew/homebrew-core#7257.

Signed-off-by: Michka Popoff <7b0496f66f66ee22a38826c310c38b415671b832@gmail.com>

rwhogg/homebrew-core,LinuxbrewTestBot/homebrew-core,maxim-belkin/homebrew-core,rwhogg/homebrew-core,LinuxbrewTestBot/homebrew-core,Linuxbrew/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core