netipmid: use the default system bus

The documentation says sd_bus_default_system() is preferred over
sd_bus_open_system() because all the calls to sd_bus_default_system()
will use the same resources, whereas the sd_bus_open_system() will
create a new connection and consume more resources for every call.

Change-Id: Ifc6a62cf9d35df3615d1133ef5c8f115ff462d98
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>
diff --git a/main.cpp b/main.cpp
index 847c991..9f61104 100644
--- a/main.cpp
+++ b/main.cpp
@@ -98,7 +98,7 @@
     ipmidbus = fopen("/dev/null", "w");
 
     // Connect to system bus
-    auto rc = sd_bus_open_system(&bus);
+    auto rc = sd_bus_default_system(&bus);
     if (rc < 0)
     {
         std::cerr << "Failed to connect to system bus:" << strerror(-rc)