committers-src: add myself (kgalazka@)

Add kgalazka@ (myself) as new src committer
with erj@ and kbowling@ as mentors.

Signed-off-by: Krzysztof Galazka <kgalazka@FreeBSD.org>
Reviewed by: kbowling, erj
Approved by: erj (mentor)
Differential Revision: https://reviews.freebsd.org/D50332