Htop: Unstage/gitignore INSTALL since it's regenerated by autogen.sh

[marcelpaulo]

When building from the git repository, autogensh regenerates INSTALL, so it should be unstaged and added to .gitignore.

[hishamhm]

@marcelpaulo Thank you for the heads up! Obrigado! :)

[marcelpaulo]

Thanks to you for htop: can't live without it !

[Explorer09]

Good work of getting rid of it. I was about to propose git-ignoring INSTALL file before you filed it here.
Just to note that this INSTALL file is from Automake, so its content changes depending on Automake version you installed.