Fix m4 failure in glean.cabal.in (#567)

Summary:
Didn't notice this because for some reason it doesn't cause a build failure, it just omitted a lot of the tests from glean.cabal.

```
$ make glean.cabal
rm -f glean.cabal
m4 -E -E -P .build/def/cxx/defs.m4 glean.cabal.in \
	| sed "/-- Copyright/a \\\n-- @""generated from glean.cabal.in\\n-- DO NO EDIT THIS FILE DIRECTLY" \
	> glean.cabal
m4:glean.cabal.in:2235: ERROR: end of file in string
chmod guo-w glean.cabal
```

Pull Request resolved: #567

Reviewed By: jjuliamolin

Differential Revision: D78481866

Pulled By: pepeiborra

fbshipit-source-id: 3d62a78ec8f79772de5cae1de81231276d1d2bee

Author: simonmar

glean.cabal.in

@@ -2232,7 +2232,7 @@ test-suite glean-snapshot-hack
         buildable: False
 
 -- The Haskell indexer tests *only* work when run using Cabal with
--- `cabal run` or `cabal test`, because Cabal puts the `hie-indexer`
+-- 'cabal run' or 'cabal test', because Cabal puts the 'hie-indexer'
 -- binary in the PATH.
 test-suite glean-snapshot-haskell
     import: fb-haskell, fb-cpp, deps, exe, haskell-indexer