kbuild: rpm-pkg: skip build dependency check on non-rpm systems

kbuild: rpm-pkg: skip build dependency check on non-rpm systems

Commit 8818039f959b ("kbuild: add ability to make source rpm buildable
using koji") added the BuildRequires: field.

Checking the build dependency is fine, but one annoyance is that
'make (bin)rpm-pkg' fails on non-rpm systems [1]. For example, Debian
provides rpmbuild via 'apt install rpm', but of course cannot meet the
requirement listed in the BuildRequires: field.

It is possible to pass RPMOPTS=--nodeps to work around it, but it is
reasonable to do it automatically.

If 'rpm -q rpm' fails, it is not an RPM-managed system. (The command
'rpm' is not installed at all, or was installed by other means.)

In that case, pass --nodeps to skip the build dependency check.

[1]: https://lore.kernel.org/linux-kbuild/Y6mkdYQYmjUz7bqV@li-4a3a4a4c-28e5-11b2-a85c-a8d192c6f089.ibm.com/

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