package/rpm: don't set openmp

package/rpm: don't set openmp

Don't set openmp to fix a build failure with codesourcery toolchain that
doesn't have OpenMP >= 4.5 indeed upstream doesn't want to remove the
build failure if the user provides --enable-openmp and OpenMP is < 4.5:
https://github.com/rpm-software-management/rpm/pull/1433

Fixes:
 - http://autobuild.buildroot.org/results/05dd945d24e8684aad6a2343ba7f6f8a7cea8349

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>