Only try and use f2fs on linux.

F2FS was already disabled on windows, but the library doesn't build for
mac either, so only try and use it on Linux hosts.

Change-Id: I8bacfa4c01c9ee13123aef6d05d17b3f9094c765
diff --git a/fastboot/Android.mk b/fastboot/Android.mk
index 44b9651..5a651f4 100644
--- a/fastboot/Android.mk
+++ b/fastboot/Android.mk
@@ -63,7 +63,7 @@
 LOCAL_STATIC_LIBRARIES += libselinux
 endif # HOST_OS != windows
 
-ifneq ($(HOST_OS),windows)
+ifeq ($(HOST_OS),linux)
 # libf2fs_dlutils_host will dlopen("libf2fs_fmt_host_dyn")
 LOCAL_CFLAGS += -DUSE_F2FS
 LOCAL_LDLIBS += -ldl