[WCNCR00152844] misc: Fix compile error if IPV6 is enabled as module

[Description]
Fix build error when IPV6 is enabled as module in kernel config:
    os/linux/gl_kal.c: In function kalGetIPv6Address:
    os/linux/gl_kal.c:4448:41: error: dereferencing pointer to
    incomplete type struct inet6_ifaddr

Test build ok with following Linux kernel versions:
3.10, 3.13, 3.18, 4.4

Feature: misc
Change-Id: Ib79024db410d9e4160a525e4ff89509da018bcb1
Signed-off-by: George Kuo <george.kuo@mediatek.com>
CR-Id: WCNCR00152844
diff --git a/os/linux/include/gl_os.h b/os/linux/include/gl_os.h
index d99b2bb..d6c38a3 100644
--- a/os/linux/include/gl_os.h
+++ b/os/linux/include/gl_os.h
@@ -196,7 +196,7 @@
 #include <linux/can/netlink.h>
 #include <net/netlink.h>
 
-#ifdef CONFIG_IPV6
+#if IS_ENABLED(CONFIG_IPV6)
 #include <linux/ipv6.h>
 #include <linux/in6.h>
 #include <net/if_inet6.h>