librsvg: enable vapi build

Otherwise gnome-chess will fail like this:

| error: Package `librsvg-2.0' not found in specified Vala API directories or GObject-Introspection GIR directories
| Compilation failed: 1 error(s), 0 warning(s)

Signed-off-by: Markus Volk <f_l_k@t-online.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>