vala: install vapigen-wrapper into /usr/bin/crosscripts and stage only that

Staging the whole /usr/bin is not correct, as it pulls in also
all the vala's cross binaries, which may be discovered by other recipes
and things will go wrong then.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>