Fix a build error on Mac OSX 10.8.3, 64-bit, GHC 7.4

Tue Sep 03 07:24:52:~/Code/haskell/psort$ sudo cabal install packer
Resolving dependencies...
Downloading packer-0.1.1...
Configuring packer-0.1.1...
Building packer-0.1.1...
Preprocessing library packer-0.1.1...
[1 of 5] Compiling Data.Packer.Endian ( Data/Packer/Endian.hs, dist/build/Data/Packer/Endian.o )
[2 of 5] Compiling Data.Packer.Internal ( Data/Packer/Internal.hs, dist/build/Data/Packer/Internal.o )
[3 of 5] Compiling Data.Packer.Unsafe ( Data/Packer/Unsafe.hs, dist/build/Data/Packer/Unsafe.o )
[4 of 5] Compiling Data.Packer.IO   ( Data/Packer/IO.hs, dist/build/Data/Packer/IO.o )
[5 of 5] Compiling Data.Packer      ( Data/Packer.hs, dist/build/Data/Packer.o )

Data/Packer.hs:288:37:
    Couldn't match expected type `Foreign.C.Types.CSize'
                with actual type `Int'
    In the third argument of `B.memcpy', namely `len'
    In the expression: B.memcpy ptr (ptr2 `plusPtr` o) len
    In the second argument of `($)', namely
      `\ ptr2 -> B.memcpy ptr (ptr2 `plusPtr` o) len'
cabal: Error: some packages failed to install:
packer-0.1.1 failed during the building phase. The exception was:
ExitFailure 1

No promises that this actually fixed it though.