Add a cast in jit_target_read_impl

Add a cast in jit_target_read_impl

We could change the signature of the function.  However, it would
require changing gdb_target_read in jit-reader.h, which is an exported
interface.  It's probably better to just add a cast in our code than to
break other people's code.

gdb/ChangeLog:

	* jit.c (jit_target_read_impl): Add cast.