Add const to the variable: needle

Summary:
Make it clear that the variable is const

Closes #139

Test Plan:
authored tested, contbuild

Reviewed By: markisaa@fb.com

Subscribers: folly-diffs@, yfeldblum

FB internal diff: D1878361

Signature: t1:1878361:1425071448:6871577ddb34f4a8dc30ea37842f9950c3825dde

folly/test/AsciiCaseInsensitiveBenchmark.cpp

@@ -80,7 +80,7 @@ string lorem_ipsum =
 "vulputate quam urna quis eros. Donec vel."
 "\n";
 
-string needle = "commodo";
+const string needle = "commodo";
 
 // legacy implementation
 struct AsciiCaseInsensitiveLegacy {