Consider upgrading to SEVERE output when an explicitly include:d symbol is skipped

[stuartmorgan]

I was converting some code from Obj-C and was confused by GLKMathRadiansToDegrees not showing up after I added it to the include list and made sure the header was in the transitive search. It took me a little while to find

[WARNING]: Skipped Function 'GLKMathRadiansToDegrees', inline functions are not supported.

in the multiple screens of ffigen output. If I've explicitly said I want a specific thing, and that thing can be exposed, I think that should be more clearly highlighted to make it easier to notice what the root problem is. (It took me several orders of magnitude longer to figure out the problem than it did to just rewrite the function in Dart once I knew I needed to.)