scanner: add facebook sdk to forbidden libraries

These are sourceavailable but not under a free license.
I made sure that this matches only the facebook sdk's from here:
https://github.com/facebook/facebook-android-sdk and not some real open
source libraries by facebook (fresco, stetho, ...). These seem to be
under a different namespace.

fdroid/fdroidserver#534

fdroidserver/scanner.py

@@ -75,6 +75,7 @@ def scan_source(build_dir, build=metadata.Build()):
             r'ouya.*sdk',
             r'libspen23',
             r'firebase',
+            r'''["']com.facebook.android['":]''',
         ]
     }