Add platform/tools/apksig project to manifest.

Bug: 29513123

default.xml

@@ -527,6 +527,7 @@
   <project path="system/webservd" name="platform/system/webservd" />
   <project path="toolchain/binutils" name="toolchain/binutils" />
   <project path="tools/adt/idea" name="platform/tools/adt/idea" groups="notdefault,tools" />
+  <project path="tools/apksig" name="platform/tools/apksig" groups="pdk,tradefed" />
   <project path="tools/base" name="platform/tools/base" groups="notdefault,tools" />
   <project path="tools/build" name="platform/tools/build" groups="notdefault,tools" />
   <project path="tools/external/fat32lib" name="platform/tools/external/fat32lib" groups="tools" />