Allow USE_GOMA=true without explicit USE_NINJA=true
Allow USE_GOMA=true without explicit USE_NINJA=true

Now ninja build is disabled only with USE_NINJA=false. So goma.mk
should fail with an error only when USE_NINJA=false is explicitly
specified.

Change-Id: I2fb913433d5c1e93ed2a9bd93bb5000cabd835a4

xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build