lib: use const in ipaddr2str

[mjstapp]

Use const in a utility api so callers can use const more.

[LabN-CI]

💚 Basic BGPD CI results: SUCCESS, 0 tests failed

Results table

_	_
Result	SUCCESS git merge/4704 2477428
Date	07/17/2019
Start	17:16:52
Finish	17:38:39
Run-Time	21:47
Total	1815
Pass	1815
Fail	0
Valgrind-Errors	0
Valgrind-Loss	0
Details	vncregress-2019-07-17-17:16:52.txt
Log	autoscript-2019-07-17-17:17:42.log.bz2
Memory	434 430 361

For details, please contact louberger

[NetDEF-CI]

Continuous Integration Result: SUCCESSFUL

Congratulations, this patch passed basic tests

Tested-by: NetDEF / OpenSourceRouting.org CI System

CI System Testrun URL: https://ci1.netdef.org/browse/FRR-FRRPULLREQ-8386/

This is a comment from an automated CI system.
For questions and feedback in regards to this CI system, please feel free to email
Martin Winter - mwinter (at) opensourcerouting.org.

Warnings Generated during build:

Debian 10 amd64 build: Successful with additional warnings

Debian Package lintian failed for Debian 10 amd64 build:
(see full package build log at https://ci1.netdef.org/browse/FRR-FRRPULLREQ-8386/artifact/DEB10BUILD/ErrorLog/log_lintian.txt)

W: frr source: changelog-should-mention-nmu

---

CLANG Static Analyzer Summary

• Github Pull Request 4704, comparing to Git base SHA 974c214

No Changes in Static Analysis warnings compared to base

1 Static Analyzer issues remaining.

See details at
https://ci1.netdef.org/browse/FRR-FRRPULLREQ-8386/artifact/shared/static_analysis/index.html