Unstyled flycheck-inline-* faces

Hi,

Thank you for maintaining this awesome package!

Recently I found that the inline error messages sometimes used the default face. It seems that is because those flycheck-inline-* faces are inheriting from the corresponding compilation-* faces, which are defined in compile.el and that library may not always be loaded.

Here is a recipe to reproduce the issue:

1. Run emacs -Q;
2. M-x package-initialize RET;
3. M-x global-flycheck-inline-mode RET;
4. M-x describe-face RET flycheck-inline-info RET.

In the resulting buffer, you can see the sample text is unstyled.

Thank you!