Add VFO_B as valid VFO for trxmanager set_split_freq_mode

https://github.com/Hamlib/Hamlib/issues/724

rigs/dummy/trxmanager.c

@@ -1157,7 +1157,7 @@ static int trxmanager_set_split_freq_mode(RIG *rig, vfo_t vfo, freq_t freq,
 
     rig_debug(RIG_DEBUG_TRACE, "%s\n", __func__);
 
-    if (vfo != RIG_VFO_CURR && vfo != RIG_VFO_TX)
+    if (vfo != RIG_VFO_CURR && vfo != RIG_VFO_TX && vfo != RIG_VFO_B)
     {
         return -RIG_ENTARGET;
     }

rigs/dummy/trxmanager.h

@@ -30,7 +30,7 @@
 #include <sys/time.h>
 #endif
 
-#define BACKEND_VER "20200329"
+#define BACKEND_VER "20210613"
 
 #define EOM "\r"
 #define TRUE 1