testsuite/switchtest: Fix help output

As the Xenomai libs hook into the option parser and hijack --help, we
need to provide application_usage() in order to get the full help
printed.

Needs eventually a better fix, i.e. the removal of the implicit parser
hook.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

testsuite/switchtest/switchtest.c

@@ -1062,6 +1062,11 @@ static void usage(FILE *fd, const char *progname)
 	fprintf(fd, "\n\n");
 }
 
+void application_usage(void)
+{
+	usage(stdout, get_program_name());
+}
+
 static sigjmp_buf jump;
 
 static void illegal_instruction(int sig)