Bugfix: do not erase BitRef::operator=(...) on non-mobile

Summary:
`FOLLY_MOBILE` is always defined, though it's 1 on mobile and 0 on non-mobile.

We only want to erase for mobile to reduce binary size, though existing code will erase both.

Differential Revision: D34577423

fbshipit-source-id: 51fa74779fbbfc41394f774abe70d406b17442ed

thrift/lib/cpp2/FieldRef.h

@@ -192,7 +192,7 @@ class BitRef {
         bit_index_(other.bit_index_),
         is_atomic_(other.is_atomic_) {}
 
-#ifdef FOLLY_MOBILE
+#if FOLLY_MOBILE
   // We have this attribute to prevent binary size regression
   // TODO: Remove special attribute for MOBILE
   FOLLY_ERASE