Work around macos case sensitivity issue

See also haskell/cabal#4739.

/Users/travis/build/PostgREST/postgrest/<built-in>:15:10: error:
     error: non-portable path to file '".stack-work/dist/x86_64-osx/Cabal-2.2.0.1/build/PostgREST/autogen/cabal_macros.h"'; specified path differs in case from file name on disk [-Werror,-Wnonportable-include-path]
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         ".stack-work/dist/x86_64-osx/Cabal-2.2.0.1/build/PostgREST/autogen/cabal_macros.h"

stack.yaml

@@ -13,6 +13,6 @@ extra-deps:
 - deferred-folds-0.9.10.1
 - primitive-0.6.4.0
 ghc-options:
-  postgrest: -O2 -Werror -Wall -fwarn-identities -fno-warn-redundant-constraints
+  postgrest: -O2 -Werror -Wall -fwarn-identities -fno-warn-redundant-constraints -optP-Wno-nonportable-include-path
 nix:
   packages: [pcre, pkgconfig, postgresql, zlib]