Fix missing namespace opening and closing.

src/mlpack/core/data/image_info_impl.hpp

@@ -51,8 +51,14 @@ inline bool ImageFormatSupported(const std::string& fileName, const bool save)
   return false;
 }
 
+} // namespace data
+} // namespace mlpack
+
 #endif // HAS_STB.
 
+namespace mlpack {
+namespace data {
+
 inline ImageInfo::ImageInfo(const size_t width,
                             const size_t height,
                             const size_t channels,