gdb: remove unnecessary struct typedef in sparc64-tdep.c

gdb: remove unnecessary struct typedef in sparc64-tdep.c

When building with clang 11, I get:

      CXX    sparc64-tdep.o
    /home/smarchi/src/binutils-gdb/gdb/sparc64-tdep.c:89:15: error: anonymous non-C-compatible type given name for linkage purposes by typedef declaration; add a tag name here [-Werror,-Wnon-c-typedef-for-linkage]
    typedef struct
                  ^
                   adi_stat_t
    /home/smarchi/src/binutils-gdb/gdb/sparc64-tdep.c:103:16: note: type is not C-compatible due to this default member initializer
      int tag_fd = 0;
                   ^
    /home/smarchi/src/binutils-gdb/gdb/sparc64-tdep.c:111:3: note: type is given name 'adi_stat_t' for linkage purposes by this typedef declaration
    } adi_stat_t;
      ^

The typedef is not needed in C++ anyway, just remove them.

gdb/ChangeLog:

	* sparc64-tdep.c (adi_stat_t): Remove typedef (leaving struct).
	(sparc64_adi_info): Likewise.