tools: testing: define the _GNU_SOURCE macro

Add the macro _GNU_SOURCE, to fix CPU_ZERO and CPU_SET undefined compile
errors.
Signed-off-by: NMuhammad Falak R Wani <falakreyaz@gmail.com>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/intel_pstate/run.sh

@@ -32,7 +32,7 @@ EVALUATE_ONLY=0
 max_cpus=$(($(nproc)-1))
 
 # compile programs
-gcc -o aperf aperf.c -lm
+gcc aperf.c -Wall -D_GNU_SOURCE -o aperf  -lm
 [ $? -ne 0 ] && echo "Problem compiling aperf.c." && exit 1
 gcc -o msr msr.c -lm
 [ $? -ne 0 ] && echo "Problem compiling msr.c." && exit 1