Avoid redefinition of mmsghdr with Android NDK unified headers
Avoid redefinition of mmsghdr with Android NDK unified headers

Summary:
Avoid redefinition of mmsghdr with newer Android NDK unified headers.

Unified headers unconditionally define `struct mmsghdr`, but the `sendmmsg` and `recvmmsg` system calls are conditional on API level. API 21 is the first level that supports them.

Reviewed By: yfeldblum

Differential Revision: D15344307

fbshipit-source-id: 6f03197cc346511ce7a02f1468c35313140a8fd7

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