.gitignore: ignore modules.order explicitly

The pattern '*.order' was added by commit c6025f4c ("kbuild: ignore
*.order files") to ignore modules.order files.

I do not see any other user of the '.order' extension.

Ignore 'modules.order' explicitly instead of '*.order'.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>