maint: Add module_block.h to .gitignore

Commit 0c0c1fd9 generated module_block.h automatically, Add it to .gitignore to
avoid checking in it by 'git add .'.

Signed-off-by: Lin Ma <lma@suse.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
