Add missing make dist of gen_dispatch.py.

Fixes distcheck when built out of tree (not that I'm using make dist
for distributing releases, currently).

src/Makefile.am

@@ -157,3 +157,7 @@ $(GENERATED_EGL_SOURCE): $(srcdir)/gen_dispatch.py $(top_srcdir)/registry/egl.xm
 
 $(GENERATED_WGL_SOURCE): $(srcdir)/gen_dispatch.py $(top_srcdir)/registry/wgl.xml
 	$(AM_V_GEN)$(PYTHON) $(srcdir)/gen_dispatch.py --dir $(top_builddir) $(top_srcdir)/registry/wgl.xml
+
+EXTRA_DIST = \
+	gen_dispatch.py \
+	$()