Add ui-shared.h

This is finally a proper headerfile for the shared ui-functions which
used to reside in cgit.h

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

ui-repolist.c

@@ -10,6 +10,7 @@
 
 #include "cgit.h"
 #include "html.h"
+#include "ui-shared.h"
 
 time_t read_agefile(char *path)
 {