Fix libtitan_sys sse flag (#315)

Fix typo done in #312

Signed-off-by: Yi Wu yiwu@pingcap.com

librocksdb_sys/Cargo.toml

@@ -17,7 +17,7 @@ default = []
 # portable doesn't require static link, though it's meaningless
 # when not using with static-link right now in this crate.
 portable = ["libtitan_sys/portable"]
-sse = ["libtitan_sys/portable"]
+sse = ["libtitan_sys/sse"]
 
 [build-dependencies]
 cc = "1.0.3"