test/stp: stub the RMA action the trap now sets

Reception moved to the RMA trap in main, so rtl837x_stp.c calls trap_rma_set() from stp_setup() and stp_off(). The simulator defines neither, so it does not link. Linking rtl837x_trap.c instead would pull trap_init() and the register primitives it needs, which the simulator does not mock.

test/stp/stp_sim.c

@@ -140,6 +140,13 @@ void port_l2mc_set(uint8_t mac_last, uint16_t vid, uint16_t pmask)
 	l2mc_pmask = pmask;
 }
 
+void trap_rma_set(uint16_t reg, uint8_t act)
+{
+	(void)reg;
+	(void)act;
+}
+
+
 uint8_t port_ingress_filter_get(uint8_t port)
 {
 	(void)port;