Use import list for `Data.Time.Clock.POSIX`

This makes it more obvious why `unix` depends on `time` in the first
place, i.e. for the sole purpose of reusing the `POSIXTime` type.

System/Posix/Files.hsc

@@ -99,7 +99,7 @@ import System.Posix.Files.Common
 import System.Posix.Error
 import System.Posix.Internals
 
-import Data.Time.Clock.POSIX
+import Data.Time.Clock.POSIX (POSIXTime)
 
 -- -----------------------------------------------------------------------------
 -- chmod()

System/Posix/Files/ByteString.hsc

@@ -104,7 +104,7 @@ import Foreign.C hiding (
 import System.Posix.Files.Common
 import System.Posix.ByteString.FilePath
 
-import Data.Time.Clock.POSIX
+import Data.Time.Clock.POSIX (POSIXTime)
 
 -- -----------------------------------------------------------------------------
 -- chmod()

System/Posix/Files/Common.hsc

@@ -92,7 +92,7 @@ import Data.Bits
 import Data.Int
 import Data.Ratio
 #endif
-import Data.Time.Clock.POSIX
+import Data.Time.Clock.POSIX (POSIXTime)
 import System.Posix.Internals
 import Foreign.C
 import Foreign.ForeignPtr