.gitignore: ignore *.rpm

Previously, *.rpm files were created under $HOME/rpmbuild/, but since
commit 8818039f ("kbuild: add ability to make source rpm buildable
using koji"), srcrpm-pkg creates the source rpm in the kernel tree
because it sets '_srcrpmdir'.

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