Add external/dtc.

BUG: 26412135

default.xml

@@ -110,6 +110,7 @@
   <project path="external/drm_gralloc" name="platform/external/drm_gralloc" groups="drm_gralloc" />
   <project path="external/drm_hwcomposer" name="platform/external/drm_hwcomposer" groups="drm_hwcomposer" />
   <project path="external/droiddriver" name="platform/external/droiddriver" groups="pdk-cw-fs,pdk-fs" />
+  <project path="external/dtc" name="platform/external/dtc" />
   <project path="external/e2fsprogs" name="platform/external/e2fsprogs" groups="pdk" />
   <project path="external/easymock" name="platform/external/easymock" groups="pdk-cw-fs,pdk-fs" />
   <project path="external/eclipse-basebuilder" name="platform/external/eclipse-basebuilder" groups="pdk-cw-fs,pdk-fs" />