Fix a `-Wnoncanonical-monoid-instances` warning

[RyanGlScott]

GHC 9.2 now includes -Wnoncanonical-monoid-instances as a part of -Wall. As such, Text.LLVM.AST produces a warning when built with 9.2:

src/Text/LLVM/AST.hs:60:3: warning: [-Wnoncanonical-monoid-instances]
    Noncanonical ‘mappend’ definition detected
    in the instance declaration for ‘Monoid Module’.
    ‘mappend’ will eventually be removed in favour of ‘(<>)’
    Either remove definition for ‘mappend’ (recommended) or define as ‘mappend = (<>)’
    See also: https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/semigroup-monoid
   |
60 |   mappend m1 m2 = m1 <> m2
   |   ^^^^^^^^^^^^^^^^^^^^^^^^

This fixes the warning by changing the implementation of mappend to mappend = (<>), per the warning's suggestion.