Revert "Use -Wno-* instead of the obsolete -fno-warn-*"

-fno-warn still works in current GHC versions and the rest of the X11
package works just fine with GHC <8, at least for now, so let's revert
this to make some users happy.

This reverts commit ec5ccda.

Fixes: #80

X11.cabal

@@ -82,7 +82,7 @@ library
   includes:           HsXlib.h, HsAllKeysyms.h
   install-includes:   HsXlib.h, XlibExtras.h, HsAllKeysyms.h
   include-dirs:       include
-  ghc-options:        -funbox-strict-fields -Wall -Wno-unused-binds
+  ghc-options:        -funbox-strict-fields -Wall -fno-warn-unused-binds
 
   if flag(pedantic)
     ghc-options:      -Werror