classes/buildhistory: fix expansion of escape sequences

OE-Core commit 259b8718a31b886f8a158aeb5de164840c9a28b2 fixed UTF-8
errors but broke decoding of escape sequences in strings (e.g.
pkg_postinst scripts had \n \t in them instead of newlines and tabs.)
We need a second call to decode() here as specifying 'string_escape' as
the second parameter won't do anything.

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