kbuild/deb-pkg: annotate libelf-dev dependency as :native

Cross compiling the x86 kernel on a non-x86 build machine produces
the following error when CONFIG_UNWINDER_ORC is enabled, regardless
of whether libelf-dev is installed or not.

  dpkg-checkbuilddeps: error: Unmet build dependencies: libelf-dev
  dpkg-buildpackage: warning: build dependencies/conflicts unsatisfied; aborting
  dpkg-buildpackage: warning: (Use -d flag to override.)

Since this is a build time dependency for a build tool, we need to
depend on the native version of libelf-dev so add the appropriate
annotation.

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