ui-shared: add cgit_print_layout_{start,end}()

These will avoid needing to call three functions to start page layout in
subsequent patches when we move the layout setup into each individual
page.

Signed-off-by: John Keeping <john@keeping.me.uk>
diff --git a/ui-shared.h b/ui-shared.h
index 652685e..246678b 100644
--- a/ui-shared.h
+++ b/ui-shared.h
@@ -54,6 +54,9 @@
 extern void cgit_submodule_link(const char *class, char *path,
 				const char *rev);
 
+extern void cgit_print_layout_start(void);
+extern void cgit_print_layout_end(void);
+
 __attribute__((format (printf,1,2)))
 extern void cgit_print_error(const char *fmt, ...);
 __attribute__((format (printf,1,0)))