bitbake: lib/bb/siggen.py: insert a colon between class and recipe name

bitbake: lib/bb/siggen.py: insert a colon between class and recipe name

before:
  virtual:nativeautomake_1.12.1.bb.do_compile
after:
  virtual:native:automake_1.12.1.bb.do_compile

This separation ensures that the key is readable, and if necessary,
parsable.

Unfortunately this invalidates previous native sstate signatures with
OE-Core - not much that can be done about that; however that occurs
frequently during the development cycle so it's par for the course.

(Bitbake rev: 5b96c32dad256090e9bda5af0f80c7dbcc90bde8)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>