Always #include corresponding .h in .c files

While doing this, remove declarations from header files where the
corresponding definition is declared "static" in order to avoid build
errors.

Also re-order existing headers in ui-*.c so that the file-specific
header always comes immediately after "cgit.h", helping with future
consistency.

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

html.c

@@ -6,6 +6,7 @@
  *   (see COPYING for full license text)
  */
 
+#include "html.h"
 #include <unistd.h>
 #include <stdio.h>
 #include <stdlib.h>