gobject-introspection: Add -Dintrospection only when GIR_MESON_OPTION is not empty

meson now does not allow unknown options which means we have to be
mindful of adding -Dintrospection unconditionally and provide an option
to recipes to set GIR_MESON_OPTION = "" and when recipe does that then
do not add -D<Dintrospection> option to meson. This will help recipes to
use meson 60.x+ just by adding

GIR_MESON_OPTION = ""

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>