syserr_initialize: delete redundant strerror() declaration

This line issues a warning on clang.  strerror is of course a part of
ISO C since its dawn.  We practically have never needed it.

error.c

@@ -2437,9 +2437,6 @@ get_syserr(int n)
 static VALUE
 syserr_initialize(int argc, VALUE *argv, VALUE self)
 {
-#if !defined(_WIN32)
-    char *strerror();
-#endif
     const char *err;
     VALUE mesg, error, func, errmsg;
     VALUE klass = rb_obj_class(self);