Fix weigh dependency

[Chris Warburton]

It's giving test errors on NixOS:

Building weigh-0.0.4...
Preprocessing library weigh-0.0.4...
[1 of 2] Compiling Weigh.GHCStats   ( src/Weigh/GHCStats.hs, dist/build/Weigh/GHCStats.o )
[2 of 2] Compiling Weigh            ( src/Weigh.hs, dist/build/Weigh.o )
In-place registering weigh-0.0.4...
Preprocessing test suite 'weigh-test' for weigh-0.0.4...
[1 of 1] Compiling Main             ( src/test/Main.hs, dist/build/weigh-test/weigh-test-tmp/Main.dyn_o )
Linking dist/build/weigh-test/weigh-test ...
running tests
Running 1 test suites...
Test suite weigh-test: RUNNING...

Case                             Allocated  GCs
integers count 0                        72    0
integers count 1                        88    0
integers count 2                       104    0
integers count 3                       120    0
integers count 10                      232    0
integers count 100                   1,672    0
integers count IO CAF 0                 80    0
integers count IO func 0                84    0
integers count IO CAF 1                 96    0
integers count IO func 1               100    0
ints count 1                            72    0
ints count 10                           72    0
ints count 1000000                      72    0
\_ -> IntegerStruct 0 0                 72    0
\x -> IntegerStruct x 0                 84    0
\x -> IntegerStruct x x                 84    0
\x -> IntegerStruct (x+1) x             92    0
\x -> IntegerStruct (x+1) (x+1)         92    0
\x -> IntegerStruct (x+1) (x+2)        100    0
\x -> HasInt x                          80    0
\x -> HasUnpacked (HasInt x)            80    0
\x -> HasPacked (HasInt x)             100    0

Check problems:
  ints count 1
    Allocated bytes exceeds 0: 72
  ints count 10
    Allocated bytes exceeds 0: 72
  ints count 1000000
    Allocated bytes exceeds 0: 72
Test suite weigh-test: FAIL
Test suite logged to: dist/test/weigh-0.0.4-weigh-test.log
0 of 1 test suites (0 of 1 test cases) passed.
builder for â??/nix/store/8cv57axpznzd6lsv6d65avrgbdx48c4g-weigh-0.0.4.drvâ?? failed with exit code 1