pw_fuzzer: Don't build if unsupported
pw_fuzzer: Don't build if unsupported

When fuzzing is not supported, pw_fuzzer creates a unit test target with
a couple pre-canned test inputs. Some devices can't fit the requested
buffer size provided for fuzz testing, and it's unlikely these fuzz
tests will be run on devices anyway.

For now, completely disable building pw_fuzzer tests on targets that
don't support fuzzing.

Bug: 195
Change-Id: I4e7eb0bf6d3fa0e064d8a1541600b9243e39a72d

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