cgit.h: move stdbool.h from ui-shared.h

Follow the Git policy of including system headers in only one place.

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

cgit.h

@@ -3,6 +3,8 @@
 
 
 #include <git-compat-util.h>
+#include <stdbool.h>
+
 #include <cache.h>
 #include <grep.h>
 #include <object.h>