Fix compiler warnings on Windows in CopyFile.c

direct.h declares _mkdir() on Windows.

Prevent future compiler warnings with -Wall and -Werror.

Bug: 26355209
Test: `mmma -j30 build/libs/host` for aosp_arm64-eng is successful.

Change-Id: I6d6e4edca98bef57d62829e9d1ef4e2a96cdad3d
diff --git a/libs/host/Android.mk b/libs/host/Android.mk
index 7eb3aa1..8a28457 100644
--- a/libs/host/Android.mk
+++ b/libs/host/Android.mk
@@ -4,6 +4,7 @@
 LOCAL_SRC_FILES:= \
     CopyFile.c
 
+LOCAL_CFLAGS := -Werror -Wall
 LOCAL_CFLAGS_darwin += -DMACOSX_RSRC
 
 LOCAL_MODULE:= libhost