[Merge] lp:~brandontschaefer/libsdl/forward-declare-1306629-fix into lp:ubuntu/libsdl2

[Scott Howard]

Review: Approve

from a user (DD maintainer of a depending package): Looks good, tested on trusty chroot to build an SDL2 package that was unable to be built without this patch. Thanks!
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https://code.launchpad.net/~brandontschaefer/libsdl/forward-declare-1306629-fix/+merge/216641
Your team Ubuntu branches is requested to review the proposed merge of lp:~brandontschaefer/libsdl/forward-declare-1306629-fix into lp:ubuntu/libsdl2.