Build dependency on librsvg

[romildo]

There is a build dependency on librsvg that is not checked. It is not mentioned on README.md either.

Without it installed building fails:

[...]
building
make flags: SHELL=/nix/store/d44582rghk696yw704sh5nbvbpnm69iv-bash-4.3-p42/bin/bash    
rm -rf ./gtk-3.0/.sass-cache
test -z "gtk-3.0/gtk.gresource gtk-3.0/gtk-contained.css gtk-3.0/gtk-contained-dark.css" || rm -f gtk-3.0/gtk.gresource gtk-3.0/gtk-contained.css gtk-3.0/gtk-contained-dark.css
sass --update --sourcemap=none ./gtk-3.0
      write ./gtk-3.0/gtk-contained-dark.css
      write ./gtk-3.0/gtk-contained.css
glib-compile-resources --sourcedir=./gtk-3.0 ./gtk-3.0/gtk.gresource.xml
failed to load "./gtk-3.0/assets/bullet-symbolic.svg": Couldn't recognize the image file format for file './gtk-3.0/assets/bullet-symbolic.svg'
./gtk-3.0/gtk.gresource.xml: Child process exited with code 1.
make: *** [Makefile:560: compile] Error 1

With librsvg installed, building succeeds.