Remove redundant includes

These are all included in git-compat-util.h (when necessary), which we
include in cgit.h.

Signed-off-by: John Keeping <john@keeping.me.uk>

html.c

@@ -8,12 +8,6 @@
 
 #include "cgit.h"
 #include "html.h"
-#include <unistd.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <stdarg.h>
-#include <string.h>
-#include <errno.h>
 
 /* Percent-encoding of each character, except: a-zA-Z0-9!$()*,./:;@- */
 static const char* url_escape_table[256] = {