Correct dependency rule to fix SPL build

Correct dependency rule to fix SPL build

Commit 4750884 introduced a change in the dependency generation which
breaks SPL, because the source files being built are not initially present
and are symlinked as part of the build.

The .depend file must depend not only on the files in the DEPS list but
also on the sources which did not contribute files to the DEPS list, since
these sources will otherwise not get a dependency and will not be built.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Wolfgang Denk <wd@denx.de>