This patch fixes the build with meson v0.39+

Running meson without this patch fails with:

> Meson encountered an error in file src/dleyna/meson.build, line 38,
> column 0: Tried to form an absolute path to a source dir. You should
> not do that but use relative paths instead.
>
> To get include path to any directory relative to the current dir do

> incdir = include_directories(dirname)