llvm: Depends on libxml2 for Linuxbrew
llvm: Depends on libxml2 for Linuxbrew

Fix the error ld: cannot find -lxml2

Closes Linuxbrew/linuxbrew#1008.

Closes Linuxbrew/linuxbrew#1014.

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

saketkc/linuxbrew,Linuxbrew/linuxbrew,SuperNEMO-DBD/cadfaelbrew,gnubila-france/linuxbrew,SuperNEMO-DBD/cadfaelbrew,sjackman/linuxbrew,SuperNEMO-DBD/cadfaelbrew,sjackman/linuxbrew,saketkc/linuxbrew,reelsense/linuxbrew,Angeldude/linuxbrew,saketkc/linuxbrew,sjackman/linuxbrew,gnubila-france/linuxbrew,Homebrew/linuxbrew,sjackman/linuxbrew,Homebrew/linuxbrew,Angeldude/linuxbrew,reelsense/linuxbrew,Angeldude/linuxbrew,Angeldude/linuxbrew,SuperNEMO-DBD/cadfaelbrew,gnubila-france/linuxbrew,reelsense/linuxbrew,gnubila-france/linuxbrew,reelsense/linuxbrew,saketkc/linuxbrew,Linuxbrew/linuxbrew,Linuxbrew/linuxbrew,Homebrew/linuxbrew,Homebrew/linuxbrew,Linuxbrew/linuxbrew