Fix for compilation on GNU/Linux (missing System.Web reference)

Without the patch, the following error appears when building :

    Making all in build
    Making all in m4
    Making all in SparkleLib
      MCS   ../bin/SparkleLib.dll
    warning CS2002: Source file `./Defines.cs' specified multiple times
    Compilation succeeded - 1 warning(s)
    Making all in SparkleLib/Git
      MCS   ../../bin/SparkleLib.Git.dll
    ./SparkleGit.cs(86,17): warning CS0436: The type `SparkleLib.Defines' conflicts with the imported type of same name'. Ignoring the imported type definition
    /home/melchips/public/SparkleShare/bin/SparkleLib.dll (Location of the symbol related to previous warning)
    Compilation succeeded - 1 warning(s)
    Making all in SparkleShare
    Making all in Linux
    Making all in Pixmaps
    Making all in icons
    Making all in ubuntu-mono-dark
    Making all in ubuntu-mono-light
    Making all in gnome
      MCS   ../../bin/SparkleShare.exe
    ./../SparkleInvite.cs(69,50): error CS0103: The name `HttpUtility' does not exist in the current context
    Compilation failed: 1 error(s), 0 warnings
    make[3]: *** [../../bin/SparkleShare.exe] Erreur 1
    make[2]: *** [all-recursive] Erreur 1
    make[1]: *** [all-recursive] Erreur 1
    make: *** [all-recursive] Erreur 1