Admit that iw is _GNU_SOURCE.

Change-Id: Ifa778f084405647b10166b3cb82d290f44780969
diff --git a/Android.mk b/Android.mk
index 20b0432..ad5c9e8 100644
--- a/Android.mk
+++ b/Android.mk
@@ -10,7 +10,7 @@
   bitrate.c wowlan.c coalesce.c roc.c p2p.c vendor.c \
   sections.c
 
-LOCAL_CFLAGS += -DCONFIG_LIBNL20
+LOCAL_CFLAGS += -D_GNU_SOURCE -DCONFIG_LIBNL20
 
 # Silence some warnings for now. Needs to be fixed upstream. b/26105799
 LOCAL_CFLAGS += -Wno-unused-parameter \