Fix the last warning and enable -Werror for external/flac.

The warning fix is a change from flac codebase itself:
https://git.xiph.org/?p=flac.git;a=commit;h=55910e61ac83405a3fa4f5fe539071c5d1bf48c2

Test: builds
Bug: 26936282
Change-Id: I50684f64762ed14760fb9e1c5f3ed4b92d3bb195
(cherry picked from commit e99475aff96cd4ced00751e456beb994ab7bc145)
2 files changed