[build] Check if git index actually exists

If iPXE is used as a git submodule then the ../.git/index file will
not exist, and the build will fail.  Fix by checking that the git
index file exists before adding it as a build dependency.

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

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