lib, zebra: add missing extern "C" {} blocks to new header files

[rwestphal]

This is necessary to fix a recent breakage that prevents the gRPC plugin from being loaded correctly.

[LabN-CI]

💚 Basic BGPD CI results: SUCCESS, 0 tests failed

Results table

_	_
Result	0
Date	0
Start	0
Finish	vncregress-2020-04-22-23:05:33.txt
Run-Time	autoscript-2020-04-22-23:06:31.log.bz2
Total	468 470 426
Pass	SUCCESS git merge/5994 31170dd
Fail	03/12/2020
Valgrind-Errors	23:05:30
Valgrind-Loss	23:31:19
Details	25:49
Log	1815
Memory	1815
SUCCESS git merge/6278 17e3820	0
04/22/2020	0
23:05:33	0
23:31:22	vncregress-2020-03-12-23:05:30.txt
25:49	autoscript-2020-03-12-23:06:25.log.bz2
1815	491 489 418
1815

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-12004/

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-12004/artifact/DEB10BUILD/ErrorLog/log_lintian.txt)

W: frr source: pkg-js-tools-test-is-missing
W: frr source: newer-standards-version 4.4.1 (current is 4.3.0)
W: frr source: pkg-js-tools-test-is-missing
W: frr source: newer-standards-version 4.4.1 (current is 4.3.0)
W: frr: changelog-file-missing-explicit-entry 6.0-2 -> 7.4-dev-20200423-00-g17e38209a-0 (missing) -> 7.4-dev-20200423-00-g17e38209a-0~deb10u1
W: frr-doc: changelog-file-missing-explicit-entry 6.0-2 -> 7.4-dev-20200423-00-g17e38209a-0 (missing) -> 7.4-dev-20200423-00-g17e38209a-0~deb10u1
W: frr-rpki-rtrlib: changelog-file-missing-explicit-entry 6.0-2 -> 7.4-dev-20200423-00-g17e38209a-0 (missing) -> 7.4-dev-20200423-00-g17e38209a-0~deb10u1
W: frr-snmp: changelog-file-missing-explicit-entry 6.0-2 -> 7.4-dev-20200423-00-g17e38209a-0 (missing) -> 7.4-dev-20200423-00-g17e38209a-0~deb10u1
W: frr-pythontools: changelog-file-missing-explicit-entry 6.0-2 -> 7.4-dev-20200423-00-g17e38209a-0 (missing) -> 7.4-dev-20200423-00-g17e38209a-0~deb10u1