package/ola: fix missing numpy

OLAs RDM test option needs NumPy.

Note that we need to pass `ac_cv_have_pymod_numpy=yes` as NumPy builds some
shared libraries and ola checks for NumPy using a host Python test program
which fails with 'wrong ELF class'.

Fixes:
http://autobuild.buildroot.net/results/750/750c9bcc753e2c1b7b05d6c43fabf4c405e15f04/
http://autobuild.buildroot.net/results/24a/24ada1e0ec695cb12028048728a9d1e2bcf75a03/
http://autobuild.buildroot.net/results/111/111c5f3eed6a5931de27a1b0b02be6e2114198e6/


.. and more.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>