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>

ui-plain.c

@@ -8,6 +8,7 @@
 
 #include <stdio.h>
 #include "cgit.h"
+#include "ui-plain.h"
 #include "html.h"
 #include "ui-shared.h"