[XForms] Small change in Form Designer code

[TG]

Hello!

While inserting xforms code in C++ project, I figured that generated 
header files have no C++ compiler hint as all the other xforms headers 
have. So I copied some bits from forms.h.

fd_printC.c at line 179

    fprintf(fn,
        "\n#if defined __cplusplus\n"
        "extern \"C\"\n"
        "{\n"
        "#endif\n");

and at line 212

    fprintf(fn,
        "\n#if defined __cplusplus\n"
        "}\n"
        "#endif\n");

That's work for me.

bye.
Thomas