xgl.py: fix prototype paring

Drop "xgl" prefix.
diff --git a/xgl.py b/xgl.py
index d84e4af..c7c03b8 100644
--- a/xgl.py
+++ b/xgl.py
@@ -898,7 +898,7 @@
     # parse proto_lines to protos
     protos = []
     for line in proto_lines:
-        first, rest = line.split(" (XGLAPI *")
+        first, rest = line.split(" (XGLAPI *xgl")
         second, third = rest.split("Type)(")
 
         # get the return type, no space before "*"