Call gdb_exit before gdb_skip_xml_test on gdb.base/catch-syscall.exp

Call gdb_exit before gdb_skip_xml_test on gdb.base/catch-syscall.exp

The gdb_skip_xml_test procedure explicitly says that it cannot be
invoked when GDB is running.  However, the testcase for "catch
syscall" is wrongly doing that, which is causing a failure on
native-extended-gdbserver tests:

  new FAIL: gdb.base/catch-syscall.exp: set tdesc filename /home/gdb-buildbot/fedora-x86-64-3/fedora-x86-64-native-extended-gdbserver-m32/build/gdb/testsuite/outputs/gdb.base/catch-syscall/trivial.xml (got interactive prompt)

This obvious commit fixes this, by calling gdb_exit before gdb_skip_xml_test.

Checked in as obvious.

gdb/testsuite/ChangeLog
2015-07-24  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.base/catch-syscall.exp: Call gdb_exit before
	gdb_skip_xml_test.