test(unit): add ff_unload_config no-op stub for test_ff_dpdk_if linkage

ff_dpdk_run() now calls ff_unload_config() at process-exit (replacing
the longstanding 'FIXME: Cleanup ff_config' comment). test_ff_dpdk_if
links lib_objs/ff_dpdk_if.o without ff_config.o, so the symbol must be
resolved locally. Stub is a no-op since none of the 7 TCs invoke
ff_dpdk_run.

11/11 binaries / 0 SKIP / 0 FAIL / valgrind 0 definite leak.

Author: coverage-leader

tests/unit/test_ff_dpdk_if.c

@@ -117,6 +117,11 @@ void ff_hardclock(void) { }
 int  ff_enable_pcap(const char *p, uint16_t s, uint8_t t)
 { (void)p;(void)s;(void)t; return 0; }
 
+/* ff_unload_config: defined in ff_config.c (we do NOT link it here);
+ * referenced from ff_dpdk_run's exit cleanup path which our 7 TCs never
+ * invoke. */
+void ff_unload_config(void) { }
+
 /* rte_timer_meta_init: F-Stack-specific patch added during DPDK 23->24
  * upgrade; not present in stock librte_timer.so. Provide a no-op stub. */
 void rte_timer_meta_init(void) { }