[rpm] Fix missing dependency to busybox-static

Busybox is not used during build time of the new hw-ramdisk tools,
but rather when the tools are run in target environment. So
make the dependency to busybox-static a run-time Requires.

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@jolla.com>
Merge pull request #3 from kjokinie/busybox-fix

[rpm] Fix missing dependency to busybox-static