Added check for PG version >= 16

Added check for PG version >= 16. Without it, the datasketches extension will not build correctly on previous versions.
diff --git a/src/common.c b/src/common.c
index 1057d88..9e8ef34 100644
--- a/src/common.c
+++ b/src/common.c
@@ -21,7 +21,12 @@
 #include <utils/builtins.h>
 
 #include "base64.h"
+
+// Since version 16 of PG, all functionality for variable-length
+// data was moved from postgres.h into the new file varatt.h
+#if PG_VERSION_NUM >= 160000
 #include "varatt.h"
+#endif
 
 PG_MODULE_MAGIC;