Convert to 64 bit timestamps in dma edge counting.

Change-Id: If6dbb578e7408ca6a8e87ad43d65d593db8d59eb
diff --git a/frc971/wpilib/dma_edge_counting.cc b/frc971/wpilib/dma_edge_counting.cc
index 8487ce9..4cf4440 100644
--- a/frc971/wpilib/dma_edge_counting.cc
+++ b/frc971/wpilib/dma_edge_counting.cc
@@ -36,7 +36,7 @@
         }
 
         if (remaining == 0) {
-          if (sample_time_ < current_sample.GetTime()) {
+          if (sample_time_ < static_cast<int64_t>(current_sample.GetTime())) {
             // If the latest DMA sample happened after we started polling, then
             // just use the values from it because they're more recent.
             for (auto &c : handlers_) {