dbus-glibc: Set --exec-prefix to fix pkgconfig file

Without this notify-osd fails to find dbus-binding-tool, since the
pkgconfig file would contain e.g.:

````
prefix=/nix/store/hxsbjbjn7g1j1cf60n228yi9wnzrl4yk-dbus-glib-0.104
exec_prefix=${prefix}
````

... and notify-osd is using `exec_prefix` to locate the binaries.
Set it to $dev to match the location of installed binaries (we have
`outputBin = "dev";`).

Issue #15074.