[build] Fix version.o dependency upon git index

[build] Fix version.o dependency upon git index

Commit 8540300 ("[build] Disable ccache for all relevant build
targets") attempted to generalise the rule for $(BIN)/version.o to
$(BIN)/version.% in order to apply the dependency to all relevant
build targets (debug objects, assembly listings, etc).

This generalisation appears to work for the ccache override
directives, but seems to cause make (at least, GNU make 4.0) to simply
ignore the dependency upon the git index.

Since version.c contains only some string constants, there is unlikely
to be a substantive need for its debug objects, assembly listings,
etc.  Restore the previous form of the dependency and accept that
hypothetical builds with e.g. DEBUG=version will not be handled
correctly.

Signed-off-by: Michael Brown <mcb30@ipxe.org>