Export `forkProcessWithUnmask` from `System.Posix.Process`

This is a follow-up to 17192d89 which missed that export.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>

System/Posix/Process.hsc

@@ -22,6 +22,7 @@ module System.Posix.Process (
     -- ** Forking and executing
 #ifdef __GLASGOW_HASKELL__
     forkProcess,
+    forkProcessWithUnmask,
 #endif
     executeFile,