Process ffi generics in rustdoc

This prevents an assertion from being tripped because the generics weren't
categorized.

Closes #10597

src/librustdoc/html/render.rs

@@ -474,6 +474,7 @@ impl DocFolder for Cache {
             clean::ImplItem(ref i)            => self.generics(&i.generics),
             clean::TyMethodItem(ref i)        => self.generics(&i.generics),
             clean::MethodItem(ref i)          => self.generics(&i.generics),
+            clean::ForeignFunctionItem(ref f) => self.generics(&f.generics),
             _ => {}
         }