Change paramter name to FB_STRINGIZE

Change paramter name to FB_STRINGIZE

Summary:
- The fbobjc codebase defines this macro as well, and to the same thing.
However, Clang complains on macro redefinitions if paramater names are
different, even if the macros expand to the same thing. Normalize the
Folly version to 'x', which is what fbobjc uses.

Test Plan:
- fbconfig -r folly && fbmake runtests
- Builds in fbobjc

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D1149985