legal-info: fix bug leading to one package not being considered

Due to excessive parenthesis, the TARGETS_LEGAL_INFO expression was
evaluated to something like this:

  toolchain-legal-info toolchain-external-legal-info busybox-legal-info zlib-legal-info))

Yes, with the last two parenthesis. This had the effect that the
zlib-legal-info rule was never called: the last package of $(TARGETS)
$(TARGET_HOST_DEPS) $(HOST_DEPS) was never added in the legal-info
information.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>