[modernize] Use inline constexpr in parameter_dims.h

Change-Id: I493843d5be30c512a69fc517c5622c48faa63088

include/ceres/internal/parameter_dims.h

diff --git a/include/ceres/internal/parameter_dims.h b/include/ceres/internal/parameter_dims.h
index b7cf935..5c806f7 100644
--- a/include/ceres/internal/parameter_dims.h
+++ b/include/ceres/internal/parameter_dims.h
@@ -85,16 +85,9 @@
     return std::array<T*, kNumParameterBlocks>{{ptr + Indices...}};
   }
 
-  static constexpr std::array<int, kNumParameterBlocks> params_{Ns...};
+  static constexpr inline std::array<int, kNumParameterBlocks> params_{Ns...};
 };
 
-// Even static constexpr member variables needs to be defined (not only
-// declared). As the ParameterDims class is tempalted this definition must
-// be in the header file.
-template <bool IsDynamic, int... Ns>
-constexpr std::array<int, ParameterDims<IsDynamic, Ns...>::kNumParameterBlocks>
-    ParameterDims<IsDynamic, Ns...>::params_;
-
 // Using declarations for static and dynamic parameter dims. This makes client
 // code easier to read.
 template <int... Ns>