fix needed for GHC 9.2+ and Servant 0.19+

[peterbecich]

With GHC 9.2+ and Servant 0.19+, errors like this occur:

src/Servant/API/BrowserHeader.hs:37:10: error:
    • Non type-variable argument
        in the constraint: HasContextEntry
                             (context .++ DefaultErrorFormatters) ErrorFormatters
      (Use FlexibleContexts to permit this)
    • In the instance declaration for
        ‘HasServer (BrowserHeader sym a :> sublayout) context’
   |
37 | instance ( KnownSymbol sym
   |          ^^^^^^^^^^^^^^^^^...

With GHC 9 and below, Servant 0.19 works.