msdk: Fix invalid return type build warning
void function cannot have return value
gst-plugins-bad/sys/msdk/gstmsdkbufferpool.c(332): warning C4098: 'gst_msdk_buffer_pool_release_buffer': ...
void function cannot have return value
gst-plugins-bad/sys/msdk/gstmsdkbufferpool.c(332): warning C4098: 'gst_msdk_buffer_pool_release_buffer': ...