rtsp-thread-pool.c: fix clang 10 warning

clang 10 is complaining about incompatible types due to the
glib typesystem.

```
../subprojects/gst-rtsp-server/gst/rtsp-server/rtsp-thread-pool.c:534:10: error: incompatible pointer types passing 'typeof ((((void *)0))) *' (aka 'void **') to parameter of type 'GThreadPool **' (aka 'struct _GThreadPool **') [-Werror,-Wincompatible-pointer-types]
```

Part-of: <gstreamer/gst-rtsp-server!145>
31 jobs for !145 with alatiera/fedora32-2 in 33 minutes and 35 seconds (queued for 8 seconds)
detached
Status Job ID Name Coverage
  Build Docker
passed #3922193
alpine amd64 manifest builder docker

00:00:41

passed #3922192
android docker

00:00:41

passed #3922190
cerbero fedora amd64 docker

00:00:39

passed #3922188
fedora amd64 docker

00:00:42

passed #3922195
gst-indent amd64 docker

00:00:42

 
  Preparation
passed #3922198
gst indent

00:00:33

passed #3922197
manifest

00:00:43

 
  Build
passed #3922216
gstreamer
build cerbero cross-android universal

00:06:07

passed #3922221
gst-ios-13.2
build cerbero cross-ios universal

00:20:05

passed #3922218
gstreamer
build cerbero cross win32

00:05:56

passed #3922219
gstreamer
build cerbero cross win64

00:04:06

passed #3922213
gstreamer
build cerbero fedora x86_64

00:05:54

passed #3922220
gst-macos-10.15
build cerbero macos x86_64

00:11:40

passed #3922205
build clang fedora x86_64

00:05:43

manual #3922211
1809 windows docker allowed to fail manual
build msys2
passed #3922200
build nodebug fedora x86_64

00:03:25

passed #3922202
build static fedora x86_64

00:03:25

passed #3922203
build static nodebug fedora x86_64

00:03:25

passed #3922252
1809 windows docker
build vs2017 amd64

00:14:58

passed #3924128
1809 windows docker
build vs2017 x86

00:10:35

failed #3922206
1809 windows docker
build vs2017 amd64

00:02:06

failed #3922209
1809 windows docker
build vs2017 x86

00:02:06

canceled #3922251
1809 windows docker
build vs2017 x86

01:22:17

 
  Test
passed #3922222
gstreamer
check fedora

00:05:02

passed #3922223
gstreamer
integration testsuites fedora 1/4

00:12:54

passed #3922224
gstreamer
integration testsuites fedora 2/4

00:12:44

passed #3922225
gstreamer
integration testsuites fedora 3/4

00:12:37

passed #3922226
gstreamer
integration testsuites fedora 4/4

00:13:11

 
  Integrate
passed #3922227
cross-android universal examples

00:08:40

passed #3922229
gst-ios-13.2
cross-ios universal examples

00:01:27

passed #3922228
documentation

00:11:21