Use named struct

This fixes Clang 11's warning: anonymous non-C-compatible type given name for linkage purposes by typedef declaration; add a tag name here [-Wnon-c-typedef-for-linkage]

Cherry-picks !169 (merged), which is still blocked by the CI.

mentioned in commit 5840fb2c

merged

While we wait, here is a question out of curiosity: The same typedef {} NAME; construction is used all over UG. Why is it necessary to change only this particular one?

The compiler complains because an element within the struct is initialized. I want it to be fixed, because it pollutes the output of the DuMuX builds.

Here is what Clang told me:

/home/gruenich/dune-uggrid/dune/uggrid/gm/gm.h:2520:15: warning: anonymous non-C-compatible type given name for linkage purposes by typedef declaration; add a tag name here [-Wnon-c-typedef-for-linkage]
typedef struct {
              ^
               GENERAL_ELEMENT
/home/gruenich/dune-uggrid/dune/uggrid/gm/gm.h:2547:27: note: type is not C-compatible due to this default member initializer
  INT mapped_inner_objt = -1;                               /* tag to objt mapping for free list*/
                          ^~
/home/gruenich/dune-uggrid/dune/uggrid/gm/gm.h:2562:3: note: type is given name 'GENERAL_ELEMENT' for linkage purposes by this typedef declaration
} GENERAL_ELEMENT;
  ^
1 warning generated.