Fix C++ build error by casting void *

Fix C++ build error by casting void *

Two recent patches breaks GDB C++ mode build,

  https://sourceware.org/ml/gdb-patches/2016-01/msg00150.html
  https://sourceware.org/ml/gdb-patches/2016-01/msg00086.html

gdb/remote.c: In function 'int remote_set_syscall_catchpoint(target_ops*, int, int, int, int, int*)':
gdb/remote.c:2036:39: error: invalid conversion from 'void*' to 'char*' [-fpermissive]
       catch_packet = xmalloc (maxpktsz);
                                       ^

gdb/thread.c: In function 'int do_captured_thread_select(ui_out*, void*)':
gdb/git/gdb/thread.c:1999:24: error: invalid conversion from 'void*' to 'const char*' [-fpermissive]
   const char *tidstr = tidstr_v;
                        ^

this patch fixes them by casting void * to the right type.

gdb:

2016-01-14  Yao Qi  <yao.qi@linaro.org>

	* remote.c (remote_set_syscall_catchpoint): Cast to char *.
	* thread.c (do_captured_thread_select): Cast to const char *.