bitbake: bitbake-diffsigs: Use 4 spaces as indent for recursecb

bitbake: bitbake-diffsigs: Use 4 spaces as indent for recursecb

It used 2 spaces as indent which wasn't clear enough, and might cause
confusions, people might think it was in wrong format.

Fixed:
$ bitbake bc-native -ccleansstate -Snone
$ bitbake bc-native -ccleansstate -Snone
$ bitbake-diffsigs tmp/stamps/x86_64-linux/bc-native/1.07.1-r0.do_cleansstate.sigdata.*

* Before:
Hash for dependent task bc/bc_1.07.1.bb.do_clean:virtual:native changed from [foo]
  Taint (by forced/invalidated task) changed from [foo]
Taint (by forced/invalidated task) changed from [foo]

* Now
Hash for dependent task bc/bc_1.07.1.bb.do_clean:virtual:native changed from [foo]
    Taint (by forced/invalidated task) changed from [foo]
Taint (by forced/invalidated task) changed from [foo]

(Bitbake rev: 5127a8d8e6d53f5f43a6ada7fd09b6b0c24ae989)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>