libxaw: Fix libxaw-dev to not depends on non-existent libxaw package

The libxaw recipe generate a binary package for each soname of libxaw
(currently libxaw6-6 and libxaw-7-7) but no libxaw so libxaw-dev
cannot depends on it.

Reported-by: Daiane Angolini <daiane.angolini@freescale.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>