add external/selinux to default manifest

Build problems were corrected in https://android-review.googlesource.com/147070
Bug: 19963152

default.xml

@@ -244,6 +244,7 @@
   <project path="external/robolectric" name="platform/external/robolectric" groups="pdk-cw-fs" />
   <project path="external/safe-iop" name="platform/external/safe-iop" groups="pdk" />
   <project path="external/scrypt" name="platform/external/scrypt" groups="pdk" />
+  <project path="external/selinux" name="platform/external/selinux" />
   <project path="external/sepolicy" name="platform/external/sepolicy" groups="pdk" />
   <project path="external/sfntly" name="platform/external/sfntly" groups="pdk-cw-fs,qcom_msm8x26" />
   <project path="external/skia" name="platform/external/skia" groups="pdk-cw-fs,qcom_msm8x26" />