st.c: include ccan/list/list.h to test build

I suspect the build failures with r51034 ("st.c: use ccan linked-list")
on ko1's CI machine was due to me forgetting to update common.mk :x

Lets see what happens when I only include ccan/list/list.h
Will followup with appropriate reverts or reinstating the
rest of r51034 along with a common.mk update as I watch CI
build.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51041 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

st.c

@@ -14,6 +14,7 @@
 #include <stdlib.h>
 #endif
 #include <string.h>
+#include "ccan/list/list.h"
 
 typedef struct st_table_entry st_table_entry;