Fix for alex-3.5.2.0 (#25623)

This INLINE pragma for alexScanUser was added in 9.12, but then I
ported the change to alex in 3.5.2.0
(https://github.com/haskell/alex/pull/262).

I didn't realize that GHC errors on duplicate INLINE pragmas, so
this ended up being a breaking change.

This change should be backported into 9.12

(cherry picked from commit a1d92378)