Improve const correctness.

See https://usfirst.collab.net/sf/tracker/do/viewArtifact/projects.wpilib/tracker.4_defects/artf4148

Change-Id: I47b0d5a91fd49e47e2c7348b0705e998ec815682

wpilibc/wpilibC++Devices/src/Timer.cpp

@@ -82,7 +82,7 @@ Timer::~Timer()
  *
  * @return Current time value for this timer in seconds
  */
-double Timer::Get()
+double Timer::Get() const
 {
 	double result;
 	double currentTime = GetFPGATimestamp();