pgcrypto: Remove inappropriate const qualifier

The function in question does not in fact ensure that the passed
argument is not changed, and the callers don't care much either.

contrib/pgcrypto/mbuf.c

@@ -136,7 +136,7 @@ mbuf_create(int len)
 }
 
 MBuf *
-mbuf_create_from_data(const uint8 *data, int len)
+mbuf_create_from_data(uint8 *data, int len)
 {
 	MBuf	   *mbuf;
 

contrib/pgcrypto/mbuf.h

@@ -77,7 +77,7 @@ struct PullFilterOps
  * Memory buffer
  */
 MBuf	   *mbuf_create(int len);
-MBuf	   *mbuf_create_from_data(const uint8 *data, int len);
+MBuf	   *mbuf_create_from_data(uint8 *data, int len);
 int			mbuf_tell(MBuf *mbuf);
 int			mbuf_avail(MBuf *mbuf);
 int			mbuf_size(MBuf *mbuf);