Fix storage deprecation warning when in C++17 mode

Fixes "error: redundant redeclaration of 'constexpr' static data member
'ghc::filesystem::path_helper_base<char_type>::preferred_separator'
[-Werror=deprecated]" when compiling with GCC 7.4 in -std=c++17 mode.

Author: jwnimmer-tri

include/ghc/filesystem.hpp

@@ -204,8 +204,10 @@ class path_helper_base
 #endif
 };
 
+#if  __cplusplus < 201703L
 template <typename char_type>
 constexpr char_type path_helper_base<char_type>::preferred_separator;
+#endif
 
 // 30.10.8 class path
 class GHC_FS_API_CLASS path