NetFn: Use api.hpp instead of api.h

Since NetFn declared in api.h has been gradually deprecated, this
submission is to use api.hpp instead of api.h

Change-Id: I7631e68bd62d15dfc7274058b2ece38e3421a661
Signed-off-by: George Liu <liuxiwei@ieisystem.com>
diff --git a/groupext.cpp b/groupext.cpp
index f61bf1f..9578b83 100644
--- a/groupext.cpp
+++ b/groupext.cpp
@@ -24,7 +24,7 @@
 void registerNetFnGroupExtFunctions()
 {
     // <Group Extension Command>
-    ipmi_register_callback(NETFUN_GRPEXT, GRPEXT_GET_GROUP_CMD, nullptr,
+    ipmi_register_callback(ipmi::netFnGroup, GRPEXT_GET_GROUP_CMD, nullptr,
                            ipmi_groupext, PRIVILEGE_USER);
 
     return;