This adds the direct dependencies introduced by TARGET_LDFLAGS
to the package's DEPENDS variable.

This was found by accidentally building rssileds on octeon, which
resulted in:

"Package rssileds is missing dependencies for the following libraries:
libnl-tiny.so"

Though the dependencies are provided when building for the
relevant targets ar71xx, ath79 and ramips, it seems more tidy to
specify them explicitly.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>