Initialize variable before using in own initialization

Initialize variable before using in own initialization

Summary:
Clang throws error because the callback refers to itself inside its lambda
function definition.  I prevent this by declaring the variable first then the
compiler does not complain when it is used within its lambda definition.

folly/test/TimeoutQueueTest.cpp:99:37: error: variable 'cb' is uninitialized
when used within its own initialization [-Werror,-Wuninitialized]

Test Plan:
fbconfig --clang --platform=gcc-4.7.1-glibc-2.14.1 --with-project-version
boost:1.51.0 folly/test/
fbmake opt
fbmake runtests_opt

Reviewed By: ldbrandy@fb.com

FB internal diff: D753061