Add Evdev_Get_Version()

BUG=chromium-os:36545
TEST=in chroot; follow the steps to see if the version number could be
  found in the library libevdev.so

  $ version=`git rev-parse HEAD`
  $ sudo emerge libevdev
  $ grep $version /usr/lib64/libevdev.so

  Binary file /usr/lib64/libevdev.so matches

Change-Id: I744a6704bcbd7d89c3b19e99d8bc0264379a9d7e
Reviewed-on: https://gerrit.chromium.org/gerrit/38446
Reviewed-by: Joseph Shyh-In Hwang <josephsih@chromium.org>
Commit-Ready: Chung-yih Wang <cywang@chromium.org>
Tested-by: Chung-yih Wang <cywang@chromium.org>
diff --git a/src/libevdev_event.c b/src/libevdev_event.c
index 11576e3..3f1e582 100644
--- a/src/libevdev_event.c
+++ b/src/libevdev_event.c
@@ -32,6 +32,10 @@
 #define SYN_DROPPED  3
 #endif
 
+/* make VCSID as version number */
+#ifndef VCSID
+#define VCSID "Unknown"
+#endif
 
 static void Event_Clear_Ev_Rel_State(EvdevPtr);
 
@@ -49,6 +53,11 @@
 
 static void Event_Get_Time(struct timeval*, bool);
 
+const char*
+Evdev_Get_Version() {
+    return VCSID;
+}
+
 /**
  * Input Device Event Property accessors
  */