gitignore: Add a few forgotten file patterns
    
    Add a few patterns to .gitignore to cover files which are generated
    by building grub ("make", "make check", "make dist") but which have
    been forgotten to add to .gitignore in the past.
    
    Signed-off-by: Hans Ulrich Niedermann <hun@n-dimensional.de>
    Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>