nvim: treesitter: Force nvim-treesitter to use a specific compiler

Required for Haskell. See
https://github.com/tree-sitter/tree-sitter-haskell#building-with-nvim-treesitter

nvim/.config/nvim/lua/treesitter.lua

@@ -26,3 +26,5 @@ require'nvim-treesitter.configs'.setup {
     },
   },
 }
+
+require'nvim-treesitter.install'.compilers = { "gcc" }