Add support-customtabs to the doc build.

b/22795013

Change-Id: I1363e5136ca15ea3089040d59de528c877fb53df
diff --git a/core/pathmap.mk b/core/pathmap.mk
index 4d9b069..97db9e6 100644
--- a/core/pathmap.mk
+++ b/core/pathmap.mk
@@ -120,7 +120,8 @@
         recommendation \
         v7/preference \
         v14/preference \
-        v17/preference-leanback
+        v17/preference-leanback \
+        customtabs
 
 #
 # A list of all source roots under frameworks/multidex.