Maybe make gcc happy

According to Jenkins, the GCC architectures were unhappy because:

    sigaltstack.c:82: warning: missing initializer
    sigaltstack.c:82: warning: (near initialization for 'oss.ss_size')

MFC after:	2 weeks
Sponsored by:	Dell EMC Isilon

tests/sys/kern/sigaltstack.c

@@ -79,7 +79,9 @@ ATF_TC_BODY(ss_onstack, tc)
 	stack_t ss = {
 		.ss_size = SIGSTKSZ,
 	};
-	stack_t oss = {0};
+	stack_t oss = {
+		.ss_size = 0,
+	};
 
 	ss.ss_sp = malloc(ss.ss_size);
 	ATF_REQUIRE(ss.ss_sp != NULL);