Fix incorrect const qualifier

Commit 7202d72787 added in passing some const qualifiers, but the one
on the postmaster_child_launch() startup_data argument was incorrect,
because the function itself modifies the pointed-to data.  This is
hidden from the compiler because of casts.  The qualifiers on the
functions called by postmaster_child_launch() are still correct.

src/backend/postmaster/launch_backend.c

@@ -229,7 +229,7 @@ PostmasterChildName(BackendType child_type)
  */
 pid_t
 postmaster_child_launch(BackendType child_type, int child_slot,
-						const void *startup_data, size_t startup_data_len,
+						void *startup_data, size_t startup_data_len,
 						ClientSocket *client_sock)
 {
 	pid_t		pid;