Add git_sha1.h to .gitignore

Add git_sha1.h to .gitignore

Since commit de579a1 "Include GIT SHA1 in GL version string"

$ git status
 On branch master
 Your branch is ahead of 'origin/master' by 2 commits.

 Untracked files:
   (use "git add <file>..." to include in what will be committed)

       src/mesa/main/git_sha1.h
nothing added to commit but untracked files present (use "git add" to track)

Add git_sha1.h to .gitignore so git knows not to warn it is present but untracked

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Marek Olšák <maraeo@gmail.com>