libnetwork: fix build with golang 1.16

Add GO111MODULE=auto to GO_PKG_BUILD_VARS to allow the package to be
built in non-module mode.

Module-aware mode will be mandatory in the next golang release.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>

utils/libnetwork/Makefile

@@ -1,7 +1,7 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=libnetwork
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 PKG_LICENSE:=Apache-2.0
 PKG_LICENSE_FILES:=LICENSE
 
@@ -38,6 +38,8 @@ Libnetwork provides a native Go implementation for connecting containers.
 The goal of libnetwork is to deliver a robust Container Network Model that provides a consistent programming interface and the required network abstractions for applications.
 endef
 
+GO_PKG_BUILD_VARS += GO111MODULE=auto
+
 define Package/libnetwork/install
 	$(call GoPackage/Package/Install/Bin,$(PKG_INSTALL_DIR))