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.h

@@ -1,7 +1,7 @@
 #ifndef HTML_H
 #define HTML_H
 
-extern int htmlfd;
+#include <stddef.h>
 
 extern void html_raw(const char *txt, size_t size);
 extern void html(const char *txt);