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

folly/Preprocessor.h

@@ -72,9 +72,9 @@
 #endif
 
 /**
- * Use FB_STRINGIZE(name) when you'd want to do what #name does inside
+ * Use FB_STRINGIZE(x) when you'd want to do what #x does inside
  * another macro expansion.
  */
-#define FB_STRINGIZE(name) #name
+#define FB_STRINGIZE(x) #x
 
 #endif // FOLLY_PREPROCESSOR_