oe-selftests: add rpm to reproducible build selftest

rpm packaging has shown good numbers wrt reproducible builds as well
when testing core-image-minimal locally:

 Reproducibility summary for rpm: same=4671 different=0 missing=0 total=4671

Thus enable it in the autobuilder to be more widely tested.

Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>