builtin.h: remove unneccessary includes

This also made it clear that a few .c files under builtin/ were
depending upon some headers but had forgotten to #include them.  Add the
missing direct includes while at it.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/check-mailmap.c

@@ -4,6 +4,7 @@
 #include "ident.h"
 #include "mailmap.h"
 #include "parse-options.h"
+#include "strbuf.h"
 #include "string-list.h"
 #include "write-or-die.h"