cmake/platform_features.cmake: fix -DENABLE_CPU_USAGE_FEATURE=OFF

[ffontaine]

Fix -DENABLE_CPU_USAGE_FEATURE=OFF to allow the user to disable CPU_USAGE and avoid the following build failure without threads:

/home/giuliobenetti/autobuild/run/instance-2/output-1/build/hawktracer-e53b07bc812c4cfe8f6253ddb48ac43de8fa74a8/lib/platform/linux/cpu_usage.c:5:10: fatal error: pthread.h: No such file or directory
    5 | #include <pthread.h>
      |          ^~~~~~~~~~~

Fixes:

• http://autobuild.buildroot.org/results/7edc29e21e441e66ad7c4df1673e506950930913

Signed-off-by: Fabrice Fontaine fontaine.fabrice@gmail.com

[loganek]

Nice catch, thanks!