Fix deprecation of non-constant type arguments 'T' in this FFI API

[dkastl]

The following warning is shown when compiling with a recent version of Flutter.

../../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/ffi-0.1.3/lib/src/allocation.dart:47:33: Info: Support for using non-constant type arguments 'T' in this FFI API is deprecated and will be removed in the next stable version of Dart. Rewrite the code to ensure that type arguments are compile time constants referring to a valid native type.
  final int totalSize = count * sizeOf<T>();                            
                                ^                         

This may break compilation in upcoming versions.