package/coremark-pro: disable parallel build

Disable parallel build as it seems to be totally broken:

/bin/bash: line 0: cd: /home/buildroot/autobuild/instance-2/output-1/build/coremark-pro-1.1.2743/builds/linux64/gcc64/obj/bench/core: No such file or directory

/bin/sh: 1: cd: can't cd to /home/buildroot/autobuild/instance-1/output-1/build/coremark-pro-1.1.2743/builds/linux/gcc/obj/bench/fp/loops/SP

Fixes:
 - http://autobuild.buildroot.org/results/7ba5e209772af7037fc735ea174d3fc3eaf46f4b
 - http://autobuild.buildroot.org/results/32b51bb9eda7899b6cc331f10a860644bd6004fa



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>