neofetch: Only depend on screenresolution on macOS
neofetch: Only depend on screenresolution on macOS

screenresolution only works on Macs and isn't needed to determine the
screen resolution on Linux anyways.

Closes Linuxbrew/homebrew-core#1035.

Signed-off-by: Bob W. Hogg <c772a964fd55352a3510e5d535dd9ccc9ac30168@linux.com>

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