Remove inclusion of postgres.h from a few header files.

Thomas Munro, per project policy articuled by Andres Freund and
Tom Lane.

Discussion: http://postgr.es/m/CAEepm=2zCoeq3QxVwhS5DFeUh=yU6z81pbWMgfOB8OzyiBwxzw@mail.gmail.com

src/include/access/bufmask.h

@@ -17,7 +17,6 @@
 #ifndef BUFMASK_H
 #define BUFMASK_H
 
-#include "postgres.h"
 #include "storage/block.h"
 #include "storage/bufmgr.h"
 

src/include/utils/dsa.h

@@ -14,8 +14,6 @@
 #ifndef DSA_H
 #define DSA_H
 
-#include "postgres.h"
-
 #include "port/atomics.h"
 #include "storage/dsm.h"