minor fix for nag compiler.

Author: jacobwilliams

src/json_value_module.F90

@@ -6863,7 +6863,7 @@ subroutine add_to_path(str,path_sep)
         !! prepend the string to the path
         implicit none
         character(kind=CK,len=*),intent(in) :: str  !! string to prepend to `path`
-        character(kind=CK,len=1),intent(in),optional :: path_sep
+        character(kind=CK,len=*),intent(in),optional :: path_sep
             !! path separator (default is '.').
             !! (ignored if `json%path_mode/=1`)