x265: disable parallel build

x265: disable parallel build

It seems package has a parallel build issue on ARM cortex A8 or A9 since
at least version 2.5:

[ 94%] Linking CXX shared library libx265.so
ipfilter8.S.o: file not recognized: File truncated
collect2: error: ld returned 1 exit status
CMakeFiles/x265-shared.dir/build.make:221: recipe for target 'libx265.so.160' failed

Fixes:
 - http://autobuild.buildroot.org/results/f6ea88324a8f9ac8ee780ddd71ec61f922e20210
 - http://autobuild.buildroot.org/results/3bd91a5694936650ce936a408ddd50338f65f8b0

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