Remove constness of libdir in do_start_initialization

Remove constness of libdir in do_start_initialization

The patch "Fix memory leak in python.c:do_start_initialization"
(https://sourceware.org/ml/gdb-patches/2017-03/msg00407.html) introduced a
compilation error on some platforms:

  ../../binutils-gdb/gdb/python/python.c: In function bool do_start_initialization():
  ../../binutils-gdb/gdb/python/python.c:1556:16: error: invalid conversion from const void* to void* [-fpermissive]
   xfree (libdir);
                ^

This is fixed by removing the constness of libdir's data type.

gdb/ChangeLog:

	* python/python.c (do_start_initialization): Remove 'const' from
	data type of libdir.