kfail gdb.python/py-evsignal.exp on RSP targets properly

Fixes, with --target_board=native-extended-gdbserver:

   Running ..../src/gdb/testsuite/gdb.python/py-evsignal.exp ...
   FAIL: gdb.python/py-evsignal.exp: signal Thread 3

gdb/testsuite/ChangeLog:
2017-10-13  Pedro Alves  <palves@redhat.com>

	* gdb.python/py-evsignal.exp: Check gdb_protocol instead of
	is_remote.