Remove redundant typedef and fix self-reference in NA_MD_LOOP

Author: kojix2

ext/numo/narray/ndloop.c

@@ -38,8 +38,6 @@ typedef struct NA_LOOP_XARGS {
     bool free_user_iter;   // alloc LARG(lp,j).iter=lp->xargs[j].iter
 } na_loop_xargs_t;
 
-typedef struct NA_MD_LOOP na_md_loop_t;
-
 typedef struct NA_MD_LOOP {
     int  narg;
     int  nin;
@@ -58,7 +56,7 @@ typedef struct NA_MD_LOOP {
     VALUE  reduce;
     VALUE  loop_opt;
     ndfunc_t  *ndfunc;
-    void (*loop_func)(ndfunc_t *, na_md_loop_t *);
+    void (*loop_func)(ndfunc_t *, struct NA_MD_LOOP *);
 } na_md_loop_t;
 
 #define LARG(lp,iarg) ((lp)->user.args[iarg])