dhcp: include "nm-dhcp-manager.h" in "nm-dhcp-listener.c"

"nm-dhcp-manager.h" forward declares _nm_dhcp_manager_factories.
We need to make the definition aware of the declaration, so
that the compiler can warn if they differ.

src/dhcp/nm-dhcp-listener.c

@@ -31,6 +31,7 @@
 
 #include "nm-dhcp-helper-api.h"
 #include "nm-dhcp-client.h"
+#include "nm-dhcp-manager.h"
 #include "nm-core-internal.h"
 #include "nm-bus-manager.h"
 #include "NetworkManagerUtils.h"