Make genvk.py work in Python2 (#894)

As a quick workaround, the Python3-specific function time.process_time()
is conditioned to the `timeit` parameter.  Indeed, if this parameter is
not set, there is no point in getting the process time anyway.

Unblocks rolling Vulkan dependencies in Google's ANGLE.

Signed-off-by: Shahbaz Youssefi <syoussefi@google.com>

xml/genvk.py

@@ -30,12 +30,13 @@ startTime = None
 
 def startTimer(timeit):
     global startTime
+    if timeit:
         startTime = time.process_time()
 
 def endTimer(timeit, msg):
     global startTime
+    if timeit:
         endTime = time.process_time()
-    if (timeit):
         write(msg, endTime - startTime, file=sys.stderr)
         startTime = None