srsenb: log cell gain updates

add a logline in info whenever the user updates the cell individual
gain. Note that this log happens before checking whether the
cell even exists and can be updated. This is mainly because
phy_common doesn't have an own logger object.

srsenb/src/phy/phy.cc

@@ -234,6 +234,7 @@ void phy::get_metrics(std::vector<phy_metrics_t>& metrics)
 
 void phy::cmd_cell_gain(uint32_t cell_id, float gain_db)
 {
+  Info("set_cell_gain: cell_id=%d, gain_db=%.2f\n", cell_id, gain_db);
   workers_common.set_cell_gain(cell_id, gain_db);
 }