proc-service.c: Add (gdb_byte *) cast

proc-service.c: Add (gdb_byte *) cast

A cast here is necessary, just as it's necessary in ps_pdwrite just
below.  The type of buf can't be changed, since it's fixed in the ps_pd*
API.

gdb/ChangeLog:

	* proc-service.c (ps_pdread): Add cast.