Clean up cpluspluscheck violation.

"operator" is a reserved word in C++, so per project conventions,
don't use it as an identifier in header files.

My oversight in commit a8081860.

src/backend/utils/adt/selfuncs.c

@@ -848,7 +848,7 @@ histogram_selectivity(VariableStatData *vardata, FmgrInfo *opproc,
  * Otherwise, fall back to the default selectivity provided by the caller.
  */
 double
-generic_restriction_selectivity(PlannerInfo *root, Oid operator,
+generic_restriction_selectivity(PlannerInfo *root, Oid oproid,
 								List *args, int varRelid,
 								double default_selectivity)
 {
@@ -886,7 +886,7 @@ generic_restriction_selectivity(PlannerInfo *root, Oid operator,
 		double		nullfrac;
 		int			hist_size;
 
-		fmgr_info(get_opcode(operator), &opproc);
+		fmgr_info(get_opcode(oproid), &opproc);
 
 		/*
 		 * Calculate the selectivity for the column's most common values.

src/include/utils/selfuncs.h

@@ -151,7 +151,7 @@ extern double histogram_selectivity(VariableStatData *vardata, FmgrInfo *opproc,
 									Datum constval, bool varonleft,
 									int min_hist_size, int n_skip,
 									int *hist_size);
-extern double generic_restriction_selectivity(PlannerInfo *root, Oid operator,
+extern double generic_restriction_selectivity(PlannerInfo *root, Oid oproid,
 											  List *args, int varRelid,
 											  double default_selectivity);
 extern double ineq_histogram_selectivity(PlannerInfo *root,