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: NLin Ma <lma@suse.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

.gitignore

@@ -55,6 +55,7 @@
 /qemu-monitor-info.texi
 /qemu-version.h
 /qemu-version.h.tmp
+/module_block.h
 /vscclient
 /fsdev/virtfs-proxy-helper
 *.[1-9]