kbuild: rpm-pkg: add libelf-devel as alternative for BuildRequires

Guoqing Jiang reports that openSUSE cannot compile the kernel rpm due
to "BuildRequires: elfutils-libelf-devel" added by commit 8818039f
("kbuild: add ability to make source rpm buildable using koji").
The relevant package name in openSUSE is libelf-devel.

Add it as an alternative package.

BTW, if it is impossible to solve the build requirement, the final
resort would be:

    $ make RPMOPTS=--nodeps rpm-pkg

This passes --nodeps to the rpmbuild command so it will not verify
build dependencies. This is useful to test rpm builds on non-rpm
system. On Debian/Ubuntu, for example, you can install rpmbuild by
'apt-get install rpm'.

NOTE1:
  Likewise, it is possible to bypass the build dependency check for
  debian package builds:

    $ make DPKG_FLAGS=-d deb-pkg

NOTE2:
  The 'or' operator is supported since RPM 4.13. So, old distros such
  as CentOS 7 will break. I suggest installing newer rpmbuild in such
  cases.

Link: https://lore.kernel.org/linux-kbuild/ee227d24-9c94-bfa3-166a-4ee6b5dfea09@linux.dev/T/#u
Fixes: 8818039f

 ("kbuild: add ability to make source rpm buildable using koji")
Reported-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Acked-by: Jonathan Toppins <jtoppins@redhat.com>