cmake: external_lib: Specify that the library should be GLOBAL

Having a library being GLOBAL, although not default behaviour, or
necessary for the sample, is expected behaviour for a library. It is
expected that like normal libraries, the target name will be
accessible from outside of the CMakeLists.txt file that created it.

Since samples are used as reference code, we specify GLOBAL so that
libraries are created with this intuitive behaviour.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>