src: move extensions into pipewire

This allows having the same directory structure for headers as it
is in $prefix/include when installed, so that we can build other
projects using pipewire uninstalled (via the -uninstalled.pc or
by using meson subprojects). Otherwise, external code that reasonably
includes <pipewire/extensions/foo.h> fails to compile.

src/meson.build

@@ -1,6 +1,5 @@
 
 subdir('pipewire')
-subdir('extensions')
 subdir('daemon')
 subdir('tools')
 subdir('modules')

src/pipewire/meson.build

@@ -120,3 +120,5 @@ pkgconfig.generate(filebase : 'lib@0@'.format(pipewire_name),
   version : pipewire_version,
   extra_cflags : '-D_REENTRANT',
   variables : ['moduledir=${libdir}/@0@'.format(pipewire_name)])
+
+subdir('extensions')