[modernize] Convert kImpossibleValue to inline constexpr in types.h

Change-Id: Ide561296f1bbcc244dd655cf229d08582281b234

include/ceres/types.h

diff --git a/include/ceres/types.h b/include/ceres/types.h
index be8e1d2..4b4f16a 100644
--- a/include/ceres/types.h
+++ b/include/ceres/types.h
@@ -472,7 +472,7 @@
 // before passing them to user code. If on return an element of the
 // array still contains this value, we will assume that the user code
 // did not write to that memory location.
-const double kImpossibleValue = 1e302;
+inline constexpr double kImpossibleValue = 1e302;
 
 CERES_EXPORT const char* LinearSolverTypeToString(LinearSolverType type);
 CERES_EXPORT bool StringToLinearSolverType(std::string value,