swr/scons: Fix intermittent build failure

gen_rasterizer*.cpp depends on gen_ar_eventhandler.hpp.
Account for new dependency.

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>

src/gallium/drivers/swr/SConscript

@@ -167,6 +167,7 @@ env.CodeGenerate(
 Depends(genRasterizerFiles,
         ['rasterizer/core/backends/gen_rasterizer.hpp',
          'rasterizer/archrast/gen_ar_event.hpp',
+         'rasterizer/archrast/gen_ar_eventhandler.hpp',
          'rasterizer/codegen/gen_knobs.h']
         )