shill: export a shared library for RTNL interface.

Export a shared library "libshill-net-<(libbase-ver)" from shill, which
currently includes the RTNL interface and other codes that it depends on
(mainly ByteString, IPAddress, IOHandler, TimeStamp, and Sockets). All the
library codes are moved to under "shill/net/".

Upcoming changes:
- Remove dependency for "shill/error.h" which depends on dbus-c++.
- Add Netlink interface to the shared library.

BUG=chromium:427982
TEST=unittests
CQ-DEPEND=CL:226103

Change-Id: I7f5c183b8a347c02e80ffed5e3497f691dde517c
Reviewed-on: https://chromium-review.googlesource.com/226110
Reviewed-by: Peter Qiu <zqiu@chromium.org>
Tested-by: Peter Qiu <zqiu@chromium.org>
Commit-Queue: Peter Qiu <zqiu@chromium.org>
diff --git a/mock_dns_client.cc b/mock_dns_client.cc
index dd28069..1b88c37 100644
--- a/mock_dns_client.cc
+++ b/mock_dns_client.cc
@@ -7,7 +7,7 @@
 #include <string>
 #include <vector>
 
-#include "shill/ip_address.h"
+#include "shill/net/ip_address.h"
 
 using std::string;
 using std::vector;