cmake: add URL to help packagers

(cherry picked from commit 0e5a96ca)

CMakeLists.txt

@@ -73,6 +73,7 @@ if (${CMAKE_SYSTEM_NAME} MATCHES "Linux")
     set_package_properties(NL PROPERTIES
         TYPE REQUIRED
         PURPOSE "Used for gathering socket info via the sock_diag netlink subsystem and for the network plugin when NetworkManagerQt is not available."
+        URL "https://github.com/thom311/libnl/"
     )
     find_package(libpcap)
     set_package_properties(