android: fix building issues of host binaries

Define _GNU_SOURCE to enable features (__USE_XOPEN2K and __USE_UNIX98)
required to build the host binaries.

Signed-off-by: Chih-Wei Huang <cwhuang@linux.org.tw>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
diff --git a/Android.common.mk b/Android.common.mk
index a4ee181..edf52d6 100644
--- a/Android.common.mk
+++ b/Android.common.mk
@@ -24,7 +24,7 @@
 # use c99 compiler by default
 ifeq ($(LOCAL_CC),)
 ifeq ($(LOCAL_IS_HOST_MODULE),true)
-LOCAL_CC := $(HOST_CC) -std=c99
+LOCAL_CC := $(HOST_CC) -std=c99 -D_GNU_SOURCE
 else
 LOCAL_CC := $(TARGET_CC) -std=c99
 endif