pdb: always set header to PDB enums, even when empty.

This fixes the following warning when generating gimpenums.h:

> Use of uninitialized value in pattern match (m//) at pdb/enumcode.pl line 71.

pdb/enums.pl

@@ -21,6 +21,7 @@ package Gimp::CodeGen::enums;
 %enums = (
     GeglDistanceMetric =>
 	{ contig => 1,
+	  header => '',
 	  external => 1,
 	  symbols => [ qw(GEGL_DISTANCE_METRIC_EUCLIDEAN
 			  GEGL_DISTANCE_METRIC_MANHATTAN