fix: add missing ghc-platform to cabal.project.stage1

ghc-boot depends on ghc-platform, so it must be included in the
stage1 build. This was incorrectly removed during rebase conflict
resolution.

Author: angerman

cabal.project.stage1

@@ -20,6 +20,7 @@ packages:
   -- Internal libraries
   libraries/ghc-boot
   libraries/ghc-boot-th-next
+  libraries/ghc-platform
   libraries/libffi-clib
 
   -- Internal tools