qtmux: Fix incompatible type warning with MSVC
gstqtmux.c(5582): warning C4133: 'function': incompatible types - from 'GstVideoMultiviewFlags *' to 'guint *'
gstqtmux.c(5582): warning C4133: 'function': incompatible types - from 'GstVideoMultiviewFlags *' to 'guint *'