Fixed compilation error on some versions of Linux.

Change-Id: I92cef7f5346215995687763569e4aabef8766800
diff --git a/y2017/vision/target_finder.cc b/y2017/vision/target_finder.cc
index 608ae75..a6b049c 100644
--- a/y2017/vision/target_finder.cc
+++ b/y2017/vision/target_finder.cc
@@ -190,7 +190,7 @@
       // 5-20, offset are around 10. Value selected for worst garbage can image.
       const double offsetWeight = 2.1;
       error += offsetWeight * offset;
-      if ((best_error < 0 || error < best_error) && !isnan(error)) {
+      if ((best_error < 0 || error < best_error) && !::std::isnan(error)) {
         best_error = error;
         best_offset = offset;
         best_v = error_v;
@@ -202,7 +202,7 @@
   }
 
   // If we missed or the error is ridiculous just give up here.
-  if (best_error < 0 || best_error > 300.0 || isnan(best_error)) {
+  if (best_error < 0 || best_error > 300.0 || ::std::isnan(best_error)) {
     fprintf(stderr, "Bogus target dude (%f).\n", best_error);
     return false;
   }