rtnetlink: Separate out gateway parsing

This same logic will get re-used by other code in a future change.

Change-Id: I28218b9defb64c15f24df989f46db255e6ce4045
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/meson.build b/src/meson.build
index 79fa0b7..ce1c142 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -52,6 +52,7 @@
   'ipaddress.cpp',
   'netlink.cpp',
   'network_manager.cpp',
+  'rtnetlink.cpp',
   'system_configuration.cpp',
   'system_queries.cpp',
   'types.cpp',