Add a missing OpenMP ifdef to coordinate_descent_minimizer.cc

Change-Id: I4eb74ab0cba86eee65a0ccfa2373c8e108f53758

internal/ceres/coordinate_descent_minimizer.cc

diff --git a/internal/ceres/coordinate_descent_minimizer.cc b/internal/ceres/coordinate_descent_minimizer.cc
index d85601d..5ce110c 100644
--- a/internal/ceres/coordinate_descent_minimizer.cc
+++ b/internal/ceres/coordinate_descent_minimizer.cc
@@ -30,6 +30,10 @@
 
 #include "ceres/coordinate_descent_minimizer.h"
 
+#ifdef CERES_USE_OPENMP
+#include <omp.h>
+#endif
+
 #include <iterator>
 #include <numeric>
 #include <vector>