Fix compilation error in ia64-vms-tdep.c with libunwind-ia64

Fix compilation error in ia64-vms-tdep.c with libunwind-ia64

I found this when doing a --enable-targets=all build with libunwind-ia64
properly configured.

  CXX    ia64-vms-tdep.o
/home/emaisin/src/binutils-gdb/gdb/ia64-vms-tdep.c: In function ‘int ia64_vms_find_proc_info_x(unw_addr_space_t, unw_word_t, unw_proc_info_t*, int, void*)’:
/home/emaisin/src/binutils-gdb/gdb/ia64-vms-tdep.c:79:33: error: invalid conversion from ‘void*’ to ‘gdb_byte* {aka unsigned char*}’ [-fpermissive]
                             pi->unwind_info, pi->unwind_info_size);
                                 ^
In file included from /home/emaisin/src/binutils-gdb/gdb/target.h:70:0,
                 from /home/emaisin/src/binutils-gdb/gdb/exec.h:23,
                 from /home/emaisin/src/binutils-gdb/gdb/gdbcore.h:29,
                 from /home/emaisin/src/binutils-gdb/gdb/ia64-vms-tdep.c:25:
/home/emaisin/src/binutils-gdb/gdb/target/target.h:35:12: note:   initializing argument 2 of ‘int target_read_memory(CORE_ADDR, gdb_byte*, ssize_t)’
 extern int target_read_memory (CORE_ADDR memaddr, gdb_byte *myaddr,
            ^

gdb/ChangeLog:

	* ia64-vms-tdep.c (ia64_vms_find_proc_info_x): Add cast.