Quote extrapackages list in pbuilder create

- Yeah, turns out if these aren't quoted pbuilder only picks up the
first one in the list.

Change-Id: I813e7b63cc457bf3c336753b3e1fb3e9b4c85cb5
diff --git a/packages.mk b/packages.mk
index f66bf24..c5eee86 100644
--- a/packages.mk
+++ b/packages.mk
@@ -31,7 +31,7 @@
 		--mirror http://ftp.debian.org/debian \
 		--distribution stretch \
 		--architecture amd64 \
-		--extrapackages crossbuild-essential-armhf crossbuild-essential-arm64 debhelper
+		--extrapackages "crossbuild-essential-armhf crossbuild-essential-arm64 debhelper"
 	mkdir -p $(ROOTDIR)/cache/base-tmp
 	cd $(ROOTDIR)/cache/base-tmp; \
 	sudo tar xf $@; \