Fix for librsvg >= 2.36.2

librsvg >= 2.36.2 now emits a warning that [including <librsvg/rsvg-cairo.h> directly is deprecated.](http://upstream-tracker.org/diffs/librsvg/2.36.1_to_2.36.2/diff.html)

With -Werror, compiles now fail :-1: 

Threw an `ifndef` around the cairo include a la [this patch to darktable](http://www.darktable.org/redmine/projects/darktable/repository/revisions/343df29553a126f29b5379fefeb7c2cfd1adc073/diff/) :+1: