package/orc: fix powerpc build with headers < 4.11

Autobuilder failures are raised with bootlin toolchains but it affects
orc since version 0.4.30

Fixes:
 - http://autobuild.buildroot.org/results/0821e96cba3e455edd47b87485501d892fc7ac6a



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>