attribute: Add a missing 'else'

The handler for the normal attribute was missing an else so presumably
it would have crashed on GLES2.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

cogl/cogl-attribute.c

@@ -703,6 +703,7 @@ flush_state (CoglDrawFlags flags,
 #ifdef HAVE_COGL_GLES2
           if (ctx->driver == COGL_DRIVER_GLES2)
             setup_generic_attribute (ctx, source, attribute, base);
+          else
 #endif
             {
               _cogl_bitmask_set (&ctx->enable_builtin_attributes_tmp,