freeimage: cleanup suggestions from code review

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

pkgs/development/libraries/freeimage/default.nix

@@ -15,8 +15,13 @@ stdenv.mkDerivation {
   sourceRoot = "svn-r1900/FreeImage/trunk";
 
   # Ensure that the bundled libraries are not used at all
-  prePatch = "rm -rf Source/Lib* Source/OpenEXR Source/ZLib";
-  patches = [ ./unbundle.diff ./libtiff-4.4.0.diff ];
+  prePatch = ''
+    rm -rf Source/Lib* Source/OpenEXR Source/ZLib
+  '';
+  patches = [
+    ./unbundle.diff
+    ./libtiff-4.4.0.diff
+  ];
 
   postPatch = ''
     # To support cross compilation, use the correct `pkg-config`.