xen/arm: build: add missed dependency for head.S

xen/arm: build: add missed dependency for head.S

When we update the header files that had been included in head.S.
The build system would not re-compile the head.S. Because in the
build rules, the dependencies are setting to .*.d (eg. DEPS = .*.d)
files in the same folder as Makefile.

But head.S is very special, it was used by the Makefile in the parent
folder: "ALL_OBJS := $(TARGET_SUBARCH)/head.o".

In this case, the build system could not find the dependency in DEPS.
When we update the header files, the build system is unware of this
update. If we re-build the Xen without doing make clean or touching
the head.S, the build system will not recompile the head.S.

Signed-off-by: Wei Chen <Wei.Chen@linaro.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>