grep API: plug memory leaks by freeing "header_list"

When the "header_list" struct member was added in [1], freeing this
field was neglected. Fix that now, so that commands like

	./git -P log -1 --color=always --author=A origin/master

will run leak-free.

1. 80235ba ("log --author=me --grep=it" should find intersection,
   not union, 2010-01-17)

Helped-by: Elijah Newren <[email protected]>
Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

grep.c

@@ -795,6 +795,7 @@ static void free_grep_pat(struct grep_pat *pattern)
 void free_grep_patterns(struct grep_opt *opt)
 {
 	free_grep_pat(opt->pattern_list);
+	free_grep_pat(opt->header_list);
 
 	if (opt->pattern_expression)
 		free_pattern_expr(opt->pattern_expression);