restic-rest-server: 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>

net/restic-rest-server/Makefile

@@ -2,7 +2,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=restic-rest-server
 PKG_VERSION:=0.9.7
-PKG_RELEASE:=2
+PKG_RELEASE:=3
 
 PKG_BUILD_DIR:=$(BUILD_DIR)/rest-server-$(PKG_VERSION)
 PKG_SOURCE:=rest-server-$(PKG_VERSION).tar.gz
@@ -43,6 +43,8 @@ API. It provides secure and efficient way to backup data remotely, using restic
 backup client via the rest: URL.
 endef
 
+GO_PKG_BUILD_VARS += GO111MODULE=auto
+
 define Package/restic-rest-server/install
 	$(INSTALL_DIR) $(1)/usr/bin
 	$(INSTALL_BIN) $(GO_PKG_BUILD_BIN_DIR)/rest-server $(1)/usr/bin/restic-rest-server