genheaders: fix time.clock() deprecation warning

> genheaders.py:78: DeprecationWarning: time.clock has been deprecated in Python 3.3 and will be removed from Python 3.8: use time.perf_counter or time.process_time instead

I chose `time.process_time()` as I assume the CPU time is what we care
about when timing the code, not the wall time that `time.perf_counter()`
returns (which includes sleeping and other processes on the system).

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
diff --git a/api/genheaders.py b/api/genheaders.py
index aef23d6..a31dca5 100755
--- a/api/genheaders.py
+++ b/api/genheaders.py
@@ -75,10 +75,10 @@
 startTime = None
 def startTimer():
     global startTime
-    startTime = time.clock()
+    startTime = time.process_time()
 def endTimer(msg):
     global startTime
-    endTime = time.clock()
+    endTime = time.process_time()
     if (timeit):
         write(msg, endTime - startTime)
         startTime = None