Add external/shflags to the manifest.

Bug: 26113731
diff --git a/default.xml b/default.xml
index 8b42de1..eda4a38 100644
--- a/default.xml
+++ b/default.xml
@@ -267,6 +267,7 @@
   <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" groups="pdk" />
+  <project path="external/shflags" name="platform/external/shflags" />
   <project path="external/sepolicy" name="platform/external/sepolicy" groups="pdk" />
   <project path="external/sfntly" name="platform/external/sfntly" groups="pdk,qcom_msm8x26" />
   <project path="external/skia" name="platform/external/skia" groups="pdk,qcom_msm8x26" />