gtk+3: add librsvg to DEPENDS

gtk+3: add librsvg to DEPENDS

adwaita-icon-themes failes to create symbolic icons. In log.do_install when
calling gtk-encode-symbolic-svg there many complaints

| Can't load file: Unrecognized image file format

We fail her now due to:

commit 7f6c57635ff4e1edf7a46c49af34846ad0a4bf73
Author: Alexander Kanavin <alex.kanavin@gmail.com>
Date:   Wed Jan 30 11:59:25 2019 +0100

gtk+3: remove the gtk-icon-utils-native recipe

gtk-icon-utils-native had librsvg in its DEPENDS. We do not add librsvg-native
because same issue would occure for gtk-encode-symbolic-svg on target.

(From OE-Core rev: d85f4e8fdffe180b54eb7e35a24a2620e5dc8d62)

Signed-off-by: Andreas Müller <schnitzeltony@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>