Add missing includes and forward declarations

I looped over the toplevel header files, creating a temporary two-line C
program for each consisting of
  #include "git-compat-util.h"
  #include $HEADER
This patch is the result of manually fixing errors in compiling those
tiny programs.

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

column.h

@@ -36,6 +36,7 @@ static inline int column_active(unsigned int colopts)
 	return (colopts & COL_ENABLE_MASK) == COL_ENABLED;
 }
 
+struct string_list;
 extern void print_columns(const struct string_list *list, unsigned int colopts,
 			  const struct column_options *opts);