travis: build gadget-zero tests as well

This helps catch some perhaps unintended api changes

tests/gadget-zero/Makefile

@@ -0,0 +1,15 @@
+# This is just a stub makefile used for travis builds
+# to keep things all compiling. Normally you'd use
+# one of the makefiles directly.
+
+# These hoops are to enable parallel make correctly.
+GZ_ALL := $(wildcard Makefile.*)
+
+all: $(GZ_ALL:=.all)
+clean: $(GZ_ALL:=.clean)
+
+%.all:
+	make -f $* all
+%.clean:
+	make -f $* clean
+