[rpmbuild] Use _smp_mflags instead of jobs macro

The "jobs" macro is non-standard and won't use multiple cores when
building under mb2.

Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>

rpm/mce-headers.spec

@@ -26,7 +26,7 @@ provided by the Mode Control Entity, and the signals emitted by it.
 
 %build
 ./verify_version
-make %{?jobs:-j%jobs}
+make %{?_smp_mflags}
 make doc
 
 %install