Remove inline declaration to fix loading problem with dyld

The `inline` declaration of `ser_free_cache` could cause undefined symbol problem with dyld of OSX.

```
[2] pry(#<RubyAMF::RequestParser>)> env['rubyamf.response'].to_s
dyld: lazy symbol binding failed: Symbol not found: _ser_free_cache
  Referenced from: /.../rbenv/.../rocketamf-3f16cf/lib/rocketamf_ext.bundle
  Expected in: flat namespace

dyld: Symbol not found: _ser_free_cache
  Referenced from: /.../rbenv/.../rocketamf-3f16cf/lib/rocketamf_ext.bundle
  Expected in: flat namespace

[1]    3576 trace trap  bundle exec rails s
```