pvr: csbgen: Add a missing @staticmethod decorator

Signed-off-by: Matt Coster <matt.coster@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16884>

src/imagination/csbgen/gen_pack_header.py

@@ -505,6 +505,7 @@ class Condition(Node):
 
         return fields
 
+    @staticmethod
     def _is_valid_type(ty):
         types = {"if", "elif", "else", "endif"}
         return ty in types