package/rt-tests: fix build issue for unsupported architectures

Release v1.9 introduced the oslat tool that only builds for supported
architectures (i386, x86_64, PPC64) and leads to a build failure for the
whole suite. An upstream patch changes this build error to a runtime warning.

Fixes:
  - http://autobuild.buildroot.net/results/72322cf7239a91211b0be74c01b55873340763f8



Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>