linux-headers: Use .gz rather than .xz
linux-headers: Use .gz rather than .xz

Fix the error:
==> Verifying git-2.7.2.tar.xz checksum
Error: Empty archive

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