Auto merge of #21427 - paulrouget:gstreamer-instructions-update, r=jdm
Auto merge of #21427 - paulrouget:gstreamer-instructions-update, r=jdm

More gstreamer dependencies

Was running into this issue:

```
error: failed to run custom build command for `gstreamer-player-sys v0.5.0`
process didn't exit successfully: `/home/paul/git/servo/target/debug/build/gstreamer-player-sys-3591c80c70153c3c/build-script-build` (exit code: 1)
--- stderr
`"pkg-config" "--libs" "--cflags" "gstreamer-player-1.0 >= 1.12"` did not exit successfully: exit code: 1
--- stderr
Package gstreamer-player-1.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `gstreamer-player-1.0.pc'
to the PKG_CONFIG_PATH environment variable
No package 'gstreamer-player-1.0' found
```

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/21427)
<!-- Reviewable:end -->

KiChjang/servo,DominoTree/servo,splav/servo,saneyuki/servo,paulrouget/servo,notriddle/servo,paulrouget/servo,larsbergstrom/servo,notriddle/servo,SimonSapin/servo,KiChjang/servo,szeged/servo,peterjoel/servo,peterjoel/servo,emilio/servo,sadmansk/servo,emilio/servo,emilio/servo,notriddle/servo,jimberlage/servo,emilio/servo,emilio/servo,sadmansk/servo,szeged/servo,KiChjang/servo,saneyuki/servo,jimberlage/servo,larsbergstrom/servo,notriddle/servo,DominoTree/servo,DominoTree/servo,jimberlage/servo,paulrouget/servo,notriddle/servo,notriddle/servo,paulrouget/servo,splav/servo,nnethercote/servo,pyfisch/servo,sadmansk/servo,sadmansk/servo,notriddle/servo,SimonSapin/servo,sadmansk/servo,notriddle/servo,larsbergstrom/servo,DominoTree/servo,pyfisch/servo,jimberlage/servo,SimonSapin/servo,larsbergstrom/servo,pyfisch/servo,SimonSapin/servo,emilio/servo,DominoTree/servo,szeged/servo,nnethercote/servo,peterjoel/servo,DominoTree/servo,paulrouget/servo,larsbergstrom/servo,DominoTree/servo,peterjoel/servo,nnethercote/servo,saneyuki/servo,paulrouget/servo,szeged/servo,pyfisch/servo,sadmansk/servo,szeged/servo,KiChjang/servo,nnethercote/servo,sadmansk/servo,peterjoel/servo,saneyuki/servo,szeged/servo,emilio/servo,splav/servo,emilio/servo,pyfisch/servo,larsbergstrom/servo,peterjoel/servo,notriddle/servo,jimberlage/servo,saneyuki/servo,szeged/servo,pyfisch/servo,DominoTree/servo,nnethercote/servo,emilio/servo,jimberlage/servo,sadmansk/servo,splav/servo,paulrouget/servo,SimonSapin/servo,splav/servo,SimonSapin/servo,KiChjang/servo,paulrouget/servo,jimberlage/servo,KiChjang/servo,saneyuki/servo,KiChjang/servo,DominoTree/servo,saneyuki/servo,saneyuki/servo,szeged/servo,splav/servo,notriddle/servo,saneyuki/servo,larsbergstrom/servo,SimonSapin/servo,DominoTree/servo,KiChjang/servo,peterjoel/servo,nnethercote/servo,SimonSapin/servo,paulrouget/servo,paulrouget/servo,larsbergstrom/servo,larsbergstrom/servo,splav/servo,nnethercote/servo,KiChjang/servo,splav/servo,pyfisch/servo,jimberlage/servo,nnethercote/servo,pyfisch/servo,KiChjang/servo,larsbergstrom/servo,szeged/servo,saneyuki/servo,pyfisch/servo,peterjoel/servo,peterjoel/servo,nnethercote/servo,peterjoel/servo,emilio/servo,jimberlage/servo,splav/servo,nnethercote/servo,pyfisch/servo,splav/servo,szeged/servo