build: Don't include Speaker type

Currently, the Speaker device is not fully implemented. So let us stop
building those files to avoid exposing them as part of the docs/gir API
until someone finishes it implementation

Part of #6

lib/meson.build

@@ -15,7 +15,6 @@ libmks_sources = [
   'mks-screen.c',
   'mks-screen-attributes.c',
   'mks-session.c',
-  'mks-speaker.c',
 ]
 
 libmks_headers = [
@@ -28,7 +27,6 @@ libmks_headers = [
   'mks-screen.h',
   'mks-screen-attributes.h',
   'mks-session.h',
-  'mks-speaker.h',
   'mks-types.h',
   'mks-version-macros.h',
 ]