Exclude verify_boot_signature from Mac build.

verify_boot_signature doesn't build on Mac currently.
Temporarily exclude it from Mac build.

Bug: 21669400
Change-Id: I7b978adafefb700d65f6fc10f69c949fce94c236
diff --git a/verity/Android.mk b/verity/Android.mk
index 46396ca..75face6 100644
--- a/verity/Android.mk
+++ b/verity/Android.mk
@@ -1,5 +1,6 @@
 LOCAL_PATH:= $(call my-dir)
 
+ifeq ($(HOST_OS),linux)
 include $(CLEAR_VARS)
 LOCAL_MODULE := verify_boot_signature
 LOCAL_SRC_FILES := verify_boot_signature.c
@@ -8,6 +9,7 @@
 LOCAL_SHARED_LIBRARIES := libcrypto-host
 LOCAL_C_INCLUDES += external/openssl/include system/extras/ext4_utils system/core/mkbootimg
 include $(BUILD_HOST_EXECUTABLE)
+endif
 
 include $(CLEAR_VARS)
 LOCAL_MODULE := generate_verity_key