Add a missing "explicit" to BiCubicInterpolator.

The constructor was missing the explicit keyword.

Change-Id: I283ab15a0d2c4cb90f85926f2aa19fa968086320

include/ceres/cubic_interpolation.h

diff --git a/include/ceres/cubic_interpolation.h b/include/ceres/cubic_interpolation.h
index 5e6b878..9d0e84c 100644
--- a/include/ceres/cubic_interpolation.h
+++ b/include/ceres/cubic_interpolation.h
@@ -246,7 +246,7 @@
 template<typename Array>
 class CERES_EXPORT BiCubicInterpolator {
  public:
-  BiCubicInterpolator(const Array& array)
+  explicit BiCubicInterpolator(const Array& array)
       : array_(array) {
     CHECK_GT(array.NumRows(), 1);
     CHECK_GT(array.NumCols(), 1);