npm.bbclass: avoid building target nodejs for native npm recipes

The current recipe unconditionally RDEPENDS on nodejs (the target one).
When building on the "-native recipe" of "BBCLASSEXTEND native" recipe,
the target nodejs is unnecessarily built.

This patch fixes this by only RDEPENDS on nodejs when building for the target.

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>