Fix missing colon in common.gypi

R=yangguo@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10912092

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12441 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

build/common.gypi

@@ -71,7 +71,7 @@
     'v8_enable_disassembler%': 0,
 
     # Enable extra checks in API functions and other strategic places.
-    'v8_enable_extra_checks%' 1,
+    'v8_enable_extra_checks%': 1,
 
     'v8_object_print%': 0,