package/qhull: force Release build

Force Release mode to always build qhull_r library instead of qhull_rd
and avoid the following build failure with python-matplotlib raised
since bump to version 3.4.3 in commit
23ca2efe and
https://github.com/matplotlib/matplotlib/commit/2dacb1c9090eda4b3c202db80f5e3ce340850760:

/home/giuliobenetti/autobuild/run/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/powerpc64-buildroot-linux-gnu/9.3.0/../../../../powerpc64-buildroot-linux-gnu/bin/ld: cannot find -lqhull_r

Fixes:
 - http://autobuild.buildroot.org/results/c2b/c2ba407275863bafe42faa4b335a20c6fa9e1d55/build-end.log



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