Add send_signal to gdbserver_tests/Makefile.am (dist_noinst_SCRIPTS).

If the send_signal script is missing the nlvgdbsigqueue test would hang.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13961 a5019735-40e9-0310-863c-91ae7b9d1cf9