rtsp-latency-bin: replace G_TYPE_INSTANCE_GET_PRIVATE as it's been deprecated

from glib
```
Deprecated: 2.58: Use %G_ADD_PRIVATE and the generated
  `your_type_get_instance_private()` function instead
```
24 jobs for get_private_deprecated in 26 minutes and 10 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #1438463
gst indent

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passed #1438462
manifest

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  Build
passed #1438473
gstreamer
build cerbero cross-android universal

00:07:38

passed #1438477
gst-ios-13.2
build cerbero cross-ios universal

00:16:23

passed #1438474
gstreamer
build cerbero cross win32

00:08:00

passed #1438475
gstreamer
build cerbero cross win64

00:05:00

passed #1438472
gstreamer
build cerbero fedora x86_64

00:05:15

passed #1438476
gst-macos-10.15
build cerbero macos x86_64

00:08:02

passed #1438468
build clang fedora x86_64

00:15:10

passed #1438464
build fedora x86_64

00:05:21

passed #1438471
docker windows 1809
build msys2

00:25:21

passed #1438465
build nodebug fedora x86_64

00:03:39

passed #1438466
build static fedora x86_64

00:11:24

passed #1438467
build static nodebug fedora x86_64

00:03:23

passed #1438469
docker windows 1809
build vs2017 amd64

00:08:45

passed #1438470
docker windows 1809
build vs2017 x86

00:08:46

 
  Test
passed #1438478
gstreamer
check fedora

00:03:42

passed #1438479
gstreamer
integration testsuites fedora 1/4

00:13:52

passed #1438480
gstreamer
integration testsuites fedora 2/4

00:14:44

passed #1438481
gstreamer
integration testsuites fedora 3/4

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passed #1438482
gstreamer
integration testsuites fedora 4/4

00:14:04

 
  Integrate
passed #1438483
cross-android universal examples

00:07:24

passed #1438485
gst-ios-13.2
cross-ios universal examples

00:04:23

passed #1438484
documentation

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