Fix `-printblocktree` output

PrintBlockTree output was broken starting from e010af70.
Everything appears on one line.

PrintWallet() added the newline after a block, but this functionality
was removed and no newline was added.

Seemingly, no one noticed. Add a newline after the block information
to fix this.
Merge pull request #3980

af4c2ac Fix `-printblocktree` output (Wladimir J. van der Laan)