tooling: Fix KEEP_SOURCES build-arg set by WITH_VTD

Author: cassava

dist/docker/Makefile

@@ -59,7 +59,7 @@ DOCKER_BUILD_ARGS += --build-arg WITH_VTD=${WITH_VTD}
 DOCKER_BUILD_ARGS += --build-arg VTD_IMAGE=${VTD_IMAGE}
 endif
 
-ifeq (${WITH_VTD},1)
+ifeq (${KEEP_SOURCES},1)
 DOCKER_IMAGE := ${DOCKER_IMAGE}-with-src
 DOCKER_BUILD_ARGS += --build-arg KEEP_SOURCES=1
 endif