msdkdec: call finalize on the parent class

Otherwise GstVideoDecoder is not finalized and resources are leaked.

Somehow GST_TRACERS="leaks" GST_DEBUG="GST_TRACER:7" did not catch it.

Valgrind output:

==31645== 22,480 (1,400 direct, 21,080 indirect) bytes in 5 blocks are definitely lost in loss record 5,042 of 5,049
==31645==    at 0x4C2FB0F: malloc
==31645==    by 0x51D9E88: g_malloc
==31645==    by 0x51FA7B5: g_slice_alloc
==31645==    by 0x51FAC68: g_slice_alloc0
==31645==    by 0x58D9984: g_type_create_instance
==31645==    by 0x58BA344: g_object_new_with_properties
==31645==    by 0x58BADA0: g_object_new
==31645==    by 0x8ECA966: gst_video_decoder_init
==31645==    by 0x58D99E7: g_type_create_instance
==31645==    by 0x58BA344: g_object_new_with_properties