meson.bbclass: export STRIP=${BUILD_STRIP}

meson.bbclass: export STRIP=${BUILD_STRIP}

In Meson the environment variables are always the native tools, so export
STRIP=${BUILD_STRIP} along with CC et al to silence this Meson warning:

WARNING: Env var STRIP seems to point to the cross compiler.
This is probably wrong, it should always point to the native compiler.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>