Add recvmmsg weak symbol.
Add recvmmsg weak symbol.

Summary:
As it turns out, to use this reliably on Android we need to define a weak symbol. The headers are only exported after API level 21, so apps that compile with 16 cannot use it with a compile time flag.

Instead we can utilize a weak symbol to link with it, and check if it's there at runtime.

Reviewed By: yfeldblum

Differential Revision: D21535034

fbshipit-source-id: 8b68a803d1e9f2f293e9f3efa16fb2a18abee53d

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