libriscv.cabal: Relax transformers version constraint

libriscv.cabal

@@ -91,7 +91,7 @@ library
         , exceptions           ^>= 0.10.4
         , melf                 ^>= 1.3.0
         , freer-simple         ^>= 1.2.1.2
-        , transformers         ^>= 0.6.1.1
+        , transformers          >= 0.5.6.0
         , optparse-applicative  >= 0.16.1 && <0.19
         , bv                   ^>= 0.5
         , parameterized-utils  ^>= 2.1.6.0