Remove a typedef that was causing a redefinition error

on some compilers

src/core/iomgr/udp_server.h

@@ -37,15 +37,16 @@
 #include "src/core/iomgr/endpoint.h"
 #include "src/core/iomgr/fd_posix.h"
 
-/* Forward decl of grpc_server */
-typedef struct grpc_server grpc_server;
+/* Forward decl of struct grpc_server */
+/* This is not typedef'ed to avoid a typedef-redefinition error */
+struct grpc_server;
 
 /* Forward decl of grpc_udp_server */
 typedef struct grpc_udp_server grpc_udp_server;
 
 /* Called when data is available to read from the socket. */
 typedef void (*grpc_udp_server_read_cb)(grpc_exec_ctx *exec_ctx, grpc_fd *emfd,
-                                        grpc_server *server);
+                                        struct grpc_server *server);
 
 /* Create a server, initially not bound to any ports */
 grpc_udp_server *grpc_udp_server_create(void);
@@ -53,7 +54,7 @@ grpc_udp_server *grpc_udp_server_create(void);
 /* Start listening to bound ports */
 void grpc_udp_server_start(grpc_exec_ctx *exec_ctx, grpc_udp_server *udp_server,
                            grpc_pollset **pollsets, size_t pollset_count,
-                           grpc_server *server);
+                           struct grpc_server *server);
 
 int grpc_udp_server_get_fd(grpc_udp_server *s, unsigned index);