libmetalink: Depends on libxml2 for Linuxbrew
libmetalink: Depends on libxml2 for Linuxbrew

Depends on either libxml2 or expat.

configure: error: Neither expat nor libxml2 found.
Please install one of them and run configure again

Closes Linuxbrew/linuxbrew#888.

Signed-off-by: Shaun Jackman <b580dab3251a9622aba3803114310c23fdb42900@gmail.com>

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