signing-keys: fix RDEPENDS to signing-keys-dev

signing-keys: fix RDEPENDS to signing-keys-dev

Specific provider (ie. RPM or IPK) does "provide" just itself as a
package to the system. This will cause the "-dev" package, which
refers to the generic signing-keys and not the provider specific,
to miss the dependency.

Error case (RPM) when populating sdk:

Error:
 Problem: conflicting requests
  - nothing provides signing-keys = 1.0-r0 needed by signing-keys-dev-1.0-r0.noarch
(try to add '--skip-broken' to skip uninstallable packages)

Examining signing-keys-rpm packages in PROVIDES we have indeed:
signing-keys-rpm = 1.0-r0

But in signing-keys-dev: in REQUIRES:
signing-keys-1.0-r0

This MR will fix the situation by removing the RDEPENDS for
the -dev package

Signed-off-by: Federico Pellegrin <fede@evolware.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>