Fix gdb.base/valgrind-infcall.exp with the native-extended-gdbserver board

Fix gdb.base/valgrind-infcall.exp with the native-extended-gdbserver board

This test fails with --target_board=native-extended-gdbserver because
it misses the usual "disconnect":

 (gdb)  target remote | /usr/lib64/valgrind/../../bin/vgdb --pid=30454
 Already connected to a remote target.  Disconnect? (y or n) n
 Still connected.
 (gdb) FAIL: gdb.base/valgrind-infcall.exp: target remote for vgdb (got interactive prompt)

gdb/testsuite/ChangeLog:
2015-08-06  Pedro Alves  <palves@redhat.com>

	* gdb.base/valgrind-infcall.exp: Issue a "disconnect".