meson: Add libpipewire dependency explicitly

This should fix being unable to be built with sandboxed build
environments (i.e., Nix package manager).

meson.build

@@ -68,6 +68,7 @@ srcs = [
 
 if get_option('wireplumber').enabled()
   deps += dependency('wireplumber-0.4')
+  deps += dependency('libpipewire-0.3')
   srcs += 'src/dbus-audiosink.c'
   srcs += 'src/wireplumber.c'
   add_project_arguments('-DWIREPLUMBER', language : 'c')