The compiling is done in a subdir, so the extraction of per-object libs
and cflags are referencing objects with ../ prefixed. So prefix the
per-object variables "foo.o-cflags" and "foo.o-libs" to
"../foo.o-cflags" and "../foo.o-libs".

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>