xgl-generate: do not hardcode #include's

Add core_headers to xgl.py and use it to generate #include's
diff --git a/xgl.py b/xgl.py
index ef358ce..58030a7 100644
--- a/xgl.py
+++ b/xgl.py
@@ -696,8 +696,11 @@
         (Param("XGL_CMD_BUFFER", "cmdBuffer"))),
 )
 
+core_headers = ("xgl.h", "xglDbg.h")
+
 # the dispatch table defined for ICDs
 # XXX figure out the real order
+# XXX this is not extensible
 icd_dispatch_table = (
     "InitAndEnumerateGpus",
     "GetGpuInfo",