gbm: Link to libwayland-drm if Wayland EGL platform is enabled

We were relying on libEGL to pull in libwayland-client symbols, but with
commit 2c2e64edaba0f6aeb181ca5b51eb8dea8e9b39f9 cleaned up the
symbol leak.

https://bugs.freedesktop.org/show_bug.cgi?id=67962
diff --git a/src/Makefile.am b/src/Makefile.am
index b3dc44d..76280a0 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -29,6 +29,10 @@
 SUBDIRS += glx
 endif
 
+if HAVE_EGL_PLATFORM_WAYLAND
+SUBDIRS += egl/wayland
+endif
+
 if HAVE_GBM
 SUBDIRS += gbm
 endif