A workaround for GHC linking problem "unknown symbol ..."

GHC 7.4 on Debian Wheezy fails to compile after some TH related changes,
reporting

  ghc: src/Ganeti/JSON.o: unknown symbol
    `attoparseczm0zi11zi1zi0_DataziAttoparsecziText_zdwf_info`

This doesn't happen on Squeeze. Explicitly importing attoparsec in THH
fixes the problem.
Signed-off-by: Petr Pudlak <pudlak@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>

src/Ganeti/THH.hs

@@ -71,6 +71,8 @@ module Ganeti.THH ( declareSADT
 import Control.Arrow ((&&&))
 import Control.Applicative
 import Control.Monad
+import Data.Attoparsec () -- Needed to prevent spurious GHC 7.4 linking errors.
+  -- See issue #683 and https://ghc.haskell.org/trac/ghc/ticket/4899
 import Data.Char
 import Data.List
 import Data.Maybe