webrtc: Fix some signals' GIR annotations

This will lead to wrong bindings otherwise (and creates more correct
expectations for developers).
13 jobs for master in 49 minutes and 38 seconds (queued for 1 minute and 27 seconds)
Status Job ID Name Coverage
  Preparation
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gst indent

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manifest

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  Build
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build fedora x86_64

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build nodebug fedora x86_64

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  Test
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check fedora

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integration testsuites fedora

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  Full Builds
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build cerbero android universal

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build cerbero cross win32

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build cerbero cross win64

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build cerbero fedora x86_64

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  Full Tests
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valgrind bad

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  Apps
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android universal examples

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  Deploy
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documentation

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