use gdb_string.h in m32c-tdep.c

use gdb_string.h in m32c-tdep.c

m32c-tdep.c is the last user of HAVE_STRING_H in gdb proper.  It
really ought to be using gdb_string.h instead, as the rest of gdb
does.

2013-11-08  Tom Tromey  <tromey@redhat.com>

	* m32c-tdep.c: Use gdb_string.h.