Fixes missing VARDATA symbol in PG16

When trying to create datasketches extension in PG16, I get an error saying that there's a missing symbol (VARDATA).
Adding an include of varatt.h fixes this issue.
diff --git a/src/common.c b/src/common.c
index f9a11f1..1057d88 100644
--- a/src/common.c
+++ b/src/common.c
@@ -21,6 +21,7 @@
 #include <utils/builtins.h>
 
 #include "base64.h"
+#include "varatt.h"
 
 PG_MODULE_MAGIC;