build: Make sure that DESTDIR is set

build: Make sure that DESTDIR is set

In the case where XEN_PYTHON_NATIVE_INSTALL is in effect,
if DESTDIR is not set then the install will go into a relative
directory rather than under the default prefix (usually /usr).

An alternate solution would be to update the fragments
that do the python install to use $(DESTDIR)/ instead of
$(DESTDIR). This is not an incredible burden as there
are only two such fragments in the tree. However, it
seems prone to error as new makefiles are created
in the future.

Signed-Off-By: Horms <horms@verge.net.au>