Two small clang-tidy fixes

Change-Id: I1eb3b5aabc9586958d618c680ca1a2c6ed501fd4

examples/slam/pose_graph_3d/types.h

diff --git a/examples/slam/pose_graph_3d/types.h b/examples/slam/pose_graph_3d/types.h
index 67b0715..4061728 100644
--- a/examples/slam/pose_graph_3d/types.h
+++ b/examples/slam/pose_graph_3d/types.h
@@ -31,6 +31,7 @@
 #ifndef EXAMPLES_CERES_TYPES_H_
 #define EXAMPLES_CERES_TYPES_H_
 
+#include <functional>
 #include <istream>
 #include <map>
 #include <string>

internal/ceres/visibility.cc

diff --git a/internal/ceres/visibility.cc b/internal/ceres/visibility.cc
index 0867733..f666ce0 100644
--- a/internal/ceres/visibility.cc
+++ b/internal/ceres/visibility.cc
@@ -88,7 +88,7 @@
   // visible to it, we find the maximum across all visibility sets.
   int num_points = 0;
   for (const auto& visible : visibility) {
-    if (visible.size() > 0) {
+    if (!visible.empty()) {
       num_points = max(num_points, (*visible.rbegin()) + 1);
     }
   }