pw_build: Always define the main facade target
pw_build: Always define the main facade target

Always define the main facade target, even when the backend is not
defined. This ensures that dependencies on the facade are still fully
expressed. Without this, libraries that depend on an unimplemented
facade may trigger gn check header inclusion errors, even if they aren't
being built.

Change-Id: I5d409c428b1587fd0858eea27490eaa4b90ea261

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed