marshal: Fix typo in the Makefile that prevented marshal .c being built
marshal: Fix typo in the Makefile that prevented marshal .c being built

pitivi/gst-editing-services,cfoch/gst-editing-services,cfoch/ges,volodymyrrudyi/gst-editing-services,cfoch/gst-editing-services,luisbg/gst-editing-services,luisbg/gst-editing-services,ahmedammar/platform_external_gst_editing_services,MathieuDuponchelle/PitiviGes,volodymyrrudyi/gst-editing-services,volodymyrrudyi/gst-editing-services,freesteph/gst-editing-services,freesteph/gst-editing-services,pitivi/gst-editing-services,MathieuDuponchelle/PitiviGes,MathieuDuponchelle/PitiviGes,cfoch/ges,freesteph/gst-editing-services,pitivi/gst-editing-services,pitivi/gst-editing-services,cfoch/ges,cfoch/ges,cfoch/gst-editing-services,ahmedammar/platform_external_gst_editing_services,MathieuDuponchelle/PitiviGes,cfoch/gst-editing-services