Fix build failure against ghc-7.4

[ 4 of 84] Compiling Graphics.Rendering.OpenGL.GL.GLboolean ( Graphics/Rendering/OpenGL/GL/GLboolean.hs, dist/build/Graphics/Rendering/OpenGL/GL/GLboolean.o )

Graphics/Rendering/OpenGL/GL/GLboolean.hs:27:23:
    Could not deduce (Eq a) arising from a use of `/='
    from the context (Num a)
      bound by the type signature for
                 unmarshalGLboolean :: Num a => a -> Bool
      at Graphics/Rendering/OpenGL/GL/GLboolean.hs:27:1-27
    Possible fix:
      add (Eq a) to the context of
        the type signature for unmarshalGLboolean :: Num a => a -> Bool
    In the expression: (/= 0)
    In an equation for `unmarshalGLboolean':
        unmarshalGLboolean = (/= 0)

Reported-by: Christian Lask
diff --git a/Graphics/Rendering/OpenGL/GL/GLboolean.hs b/Graphics/Rendering/OpenGL/GL/GLboolean.hs
index cab7356..cc4faa8 100644
--- a/Graphics/Rendering/OpenGL/GL/GLboolean.hs
+++ b/Graphics/Rendering/OpenGL/GL/GLboolean.hs
@@ -23,5 +23,5 @@ marshalGLboolean :: Num a => Bool -> a
 marshalGLboolean False = 0
 marshalGLboolean True  = 1
 
-unmarshalGLboolean :: Num a => a -> Bool
+unmarshalGLboolean :: (Eq a, Num a) => a -> Bool
 unmarshalGLboolean = (/= 0)