autotools: s/ssse3/sse3/ typo

Reported-by: Justin Erenkrantz <justin@erenkrantz.com>
Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1569 from dachary/wip-ssse3

autotools: s/ssse3/sse3/ typo

Reviewed-by: Sage Weil <sage@inktank.com>