[asan] Fix build with Android NDK < 14.

NDK < 13 & API_LEVEL < 21 do not define struct mmsghdr.
Newer NDK use unified headers and provide this definition for all api
levels.

Since we can not check for the NDK version, check the api level. This
is more strict than absolutely necessary, but it does not really
matter: it is only a sanity check.

llvm-svn: 321817