Fix Chrome OS build of update_engine.

Fix a typo in image_properties_chromeos.cc, $ifdef out an unused
constant in Chrome OS and adds the bspatch library to the Chrome OS
build.

Bug: None
Test: FEATURES=test emerge-link update_engine
Change-Id: I483da4b43b5cbd8b7349b0ff42973f97d197ba8e
diff --git a/update_engine.gyp b/update_engine.gyp
index 8ca46c1..9f63f01 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -150,6 +150,7 @@
           ],
         },
         'libraries': [
+          '-lbspatch',
           '-lbz2',
           '-lrt',
         ],