Add braces in netaddress.{h|cpp}

Summary: As per title.

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien, jasonbcox

Reviewed By: #bitcoin_abc, Fabien, jasonbcox

Subscribers: Fabien, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2663