Add cast to appease clang-armv7-linux-build-cache (D69471 followup)

http://lab.llvm.org:8011/builders/clang-armv7-linux-build-cache/builds/27075

error: non-constant-expression cannot be narrowed from type 'uint64_t'
(aka 'unsigned long long') to 'size_t' (aka 'unsigned int') in
initializer list [-Wc++11-narrowing]
  return {MappingBuf, getDataSize<FuncRecordTy, Endian>(Record)};