Update wpilib_interface.

Change-Id: If0e00a97faf0653a43e9acff9a8cff6ce3d3e6cb
diff --git a/frc971/wpilib/dma_edge_counting.cc b/frc971/wpilib/dma_edge_counting.cc
index 4b0349c..77d84b5 100644
--- a/frc971/wpilib/dma_edge_counting.cc
+++ b/frc971/wpilib/dma_edge_counting.cc
@@ -5,7 +5,7 @@
 namespace frc971 {
 namespace wpilib {
 
-bool DMAEdgeCounter::ExtractValue(const DMASample &sample) {
+bool DMAEdgeCounter::ExtractValue(const DMASample &sample) const {
   if (inverted_) {
     return !sample.Get(input_);
   } else {