Split macros from visibilitymap.h into a separate header

That allows to include just visibilitymapdefs.h from file.c, and in turn,
remove include of postgres.h from relcache.h.

Reported-by: Andres Freund
Discussion: https://postgr.es/m/20210913232614.czafiubr435l6egi%40alap3.anarazel.de
Author: Alexander Korotkov
Reviewed-by: Andres Freund, Tom Lane, Alvaro Herrera
Backpatch-through: 13

src/include/access/visibilitymapdefs.h

@@ -0,0 +1,25 @@
+/*-------------------------------------------------------------------------
+ *
+ * visibilitymapdefs.h
+ *		macros for accessing contents of visibility map pages
+ *
+ *
+ * Copyright (c) 2021, PostgreSQL Global Development Group
+ *
+ * src/include/access/visibilitymapdefs.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef VISIBILITYMAPDEFS_H
+#define VISIBILITYMAPDEFS_H
+
+/* Number of bits for one heap page */
+#define BITS_PER_HEAPBLOCK 2
+
+/* Flags for bit map */
+#define VISIBILITYMAP_ALL_VISIBLE	0x01
+#define VISIBILITYMAP_ALL_FROZEN	0x02
+#define VISIBILITYMAP_VALID_BITS	0x03	/* OR of all valid visibilitymap
+											 * flags bits */
+
+#endif							/* VISIBILITYMAPDEFS_H */