Fix builddir != srcdir

Fix builddir != srcdir

The gdkprivate-wayland.h header file is included from the top-level gdk
directory; this means that all included files referenced in the header
must be relative to the `gdk` directory, otherwise the build will fail
when the build directory is not equal to the source directory.

This commit fixes a build failure under continuous:

In file included from ../../gdk/gdkdisplaymanager.c:60:0:
../../gdk/wayland/gdkprivate-wayland.h:40:51: fatal error:
gtk-primary-selection-client-protocol.h: No such file or directory
 #include "gtk-primary-selection-client-protocol.h"
                                                   ^
compilation terminated.
Makefile:1155: recipe for target 'libgdk_3_la-gdkdisplaymanager.lo' failed
make[4]: *** [libgdk_3_la-gdkdisplaymanager.lo] Error 1