selftests/bpf: fix test_sysctl_prog with alu32

Similar to commit b7a0d65d ("bpf, testing: Workaround a verifier failure for test_progs")
fix test_sysctl_prog.c as well.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/test_sysctl_prog.c

@@ -45,7 +45,7 @@ int sysctl_tcp_mem(struct bpf_sysctl *ctx)
 	unsigned long tcp_mem[3] = {0, 0, 0};
 	char value[MAX_VALUE_STR_LEN];
 	unsigned char i, off = 0;
-	int ret;
+	volatile int ret;
 
 	if (ctx->write)
 		return 0;