build: Fix wayland-protocols.pc when using autotools

"datadir" is not the same thing in meson and autotools.

In autoconf "datadir" is "${datarootdir}", which expands to
"${prefix}/share". @datarootdir@ expands to "${prefix}/share". There
seems to be no variable that expands to "share".

In meson "datadir" is "share".

So, avoid the "datadir" variable, just expand "datarootdir" it manually
instead. This unbreaks the recently broken autotools setup.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Simon Ser <contact@emersion.fr>