[build] Make version.o depend on the git index

The version number string is currently updated only if version.o
happens to be rebuilt due to changes in its dependencies.  Add a
dependency upon the git index, so that the version number is updated
after any checkout.

Signed-off-by: Thomas Miletich <thomas.miletich@gmail.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

 		  -DVERSION_MINOR=$(VERSION_MINOR) \
 		  -DVERSION_PATCH=$(VERSION_PATCH) \
 		  -DVERSION="\"$(VERSION)\""
+# Make sure the version number gets updated on every git checkout
+ifneq ($(GITVERSION),)
+$(BIN)/version.o : ../.git/index
+endif
 
 # We automatically generate rules for any file mentioned in AUTO_SRCS
 # using the following set of templates.  It would be cleaner to use