remote.c: Add missing cast

Fixes in C++:

/home/emaisin/src/binutils-gdb/gdb/remote.c: In function ‘void start_thread(gdb_xml_parser*, const gdb_xml_element*, void*, VEC_gdb_xml_value_s*)’:
/home/emaisin/src/binutils-gdb/gdb/remote.c:2975:59: error: invalid conversion from ‘void*’ to ‘const char*’ [-fpermissive]
   item.name = attr != NULL ? (char *) xstrdup (attr->value) : NULL;
                                                           ^
In file included from /home/emaisin/src/binutils-gdb/gdb/common/common-defs.h:64:0,
                 from /home/emaisin/src/binutils-gdb/gdb/defs.h:28,
                 from /home/emaisin/src/binutils-gdb/gdb/remote.c:22:
/home/emaisin/src/binutils-gdb/gdb/../include/libiberty.h:323:14: error:   initializing argument 1 of ‘char* xstrdup(const char*)’ [-fpermissive]
 extern char *xstrdup (const char *) ATTRIBUTE_MALLOC ATTRIBUTE_RETURNS_NONNULL;
              ^
make[2]: *** [remote.o] Error 1

gdb/ChangeLog:

	* remote.c (start_thread): Add cast.