face: pass processEvents duration by value

refs #3284

Change-Id: I146417f49c73fe93c6b38bb0dcd1b45bb949511f
diff --git a/src/face.cpp b/src/face.cpp
index 2f80754..1add5db 100644
--- a/src/face.cpp
+++ b/src/face.cpp
@@ -346,7 +346,7 @@
 }
 
 void
-Face::doProcessEvents(const time::milliseconds& timeout, bool keepThread)
+Face::doProcessEvents(time::milliseconds timeout, bool keepThread)
 {
   if (m_ioService.stopped()) {
     m_ioService.reset(); // ensure that run()/poll() will do some work