.gitignore: ignore only top-level modules.builtin

modules.builtin used to be created in every directory.

Since commit 8b41fc44

 ("kbuild: create modules.builtin without
Makefile.modbuiltin or tristate.conf"), modules.builtin is created only
in the top directory.

Add the '/' prefix so that it matches to only the modules.builtin located
in the top directory.

It has been more than one year since that change. I hope this will not
flood 'Untracked files' of 'git status'.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>