Add toolchain/binutils to the platform manifest.

Needed for building libLTO and libLLVMgold for clang, which are needed
both for LTO and for using the CFI sanitizer.
Bug: http://b/21616856

default.xml

@@ -507,6 +507,7 @@
   <project path="system/vold" name="platform/system/vold" groups="pdk" />
   <project path="system/weaved" name="platform/system/weaved" />
   <project path="system/webservd" name="platform/system/webservd" />
+  <project path="toolchain/binutils" name="toolchain/binutils" />
   <project path="tools/adt/eclipse" name="platform/tools/adt/eclipse" groups="notdefault,tools" />
   <project path="tools/adt/idea" name="platform/tools/adt/idea" groups="notdefault,tools" />
   <project path="tools/base" name="platform/tools/base" groups="notdefault,tools" />