Fix remote-sim.c build after warn-unused-result change

Fix remote-sim.c build after warn-unused-result change

John Darrington pointed out that commit 18cb7c9f3 ("Introduce
ATTRIBUTE_UNUSED_RESULT and use it") broke the build:

/home/john/binutils-gdb/gdb/remote-sim.c: In function 'void gdbsim_target_open(const char*, int)':
/home/john/binutils-gdb/gdb/remote-sim.c:765:18: error: ignoring return value of 'char** gdb_argv::release()', declared with attribute warn_unused_result [-Werror=unused-result]

This patch fixes the problem by arranging to use the result of the
release method.

Tested by rebuilding with a simulator enabled.

gdb/ChangeLog
2019-03-06  Tom Tromey  <tromey@adacore.com>

	* remote-sim.c (gdbsim_target_open): Use result of
	gdb_argv::release.