kbuild: deb-pkg: hard-code Build-Depends

The condition to require libelf-dev:native is stale because objtool is
now enabled by CONFIG_OBJTOOL instead of CONFIG_UNWINDER_ORC. Not only
objtool but also resolve_btfids requires libelf-dev:native; therefore,
CONFIG_DEBUG_INFO_BTF should be checked as well.

Similarly, CONFIG_SYSTEM_TRUSTED_KEYRING is not the only case that
requires libssl-dev:native.

Perhaps, the following code would provide better coverage, but it is
hard to maintain (and may still be imperfect).

  if is_enabled CONFIG_OBJTOOL ||
     is_enabled CONFIG_DEBUG_INFO_BTF; then
          build_depends="${build_depends}, libelf-dev:native"
  fi

  if is_enabled CONFIG_SYSTEM_TRUSTED_KEYRING ||
     is_enabled CONFIG_SYSTEM_REVOCATION_LIST ||
     is_enabled CONFIG_MODULE_SIG_FORMAT; then
          build_depends="${build_depends}, libssl-dev:native"
  fi

Let's hard-code the build dependency.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>