Expand ${pkgroot} in dynamic-library-dirs

Looks like this was forgotten - it is done in GHC but not in Cabal.

(cherry picked from commit cbdb135)

Author: simonmar

Cabal/Distribution/Simple/Program/HcPkg.hs

@@ -291,6 +291,7 @@ mungePackagePaths pkgroot pkginfo =
       importDirs        = mungePaths (importDirs  pkginfo),
       includeDirs       = mungePaths (includeDirs pkginfo),
       libraryDirs       = mungePaths (libraryDirs pkginfo),
+      libraryDynDirs    = mungePaths (libraryDynDirs pkginfo),
       frameworkDirs     = mungePaths (frameworkDirs pkginfo),
       haddockInterfaces = mungePaths (haddockInterfaces pkginfo),
       haddockHTMLs      = mungeUrls  (haddockHTMLs pkginfo)