Makefile: depends.mk is not an optional include

We really need depends.mk in the following Makefile reorganization.

Signed-off-by: Kai Krakow <kai@kaishome.de>

test/Makefile

@@ -21,7 +21,7 @@ depends.mk: *.cc
 	for x in *.cc; do $(CXX) $(CXXFLAGS) -M "$$x"; done > depends.mk.new
 	mv -fv depends.mk.new depends.mk
 
--include depends.mk
+include depends.mk
 
 %.o: %.cc %.h ../makeflags
 	-echo "Implicit rule %.o: %.cc" >&2