avatar: Add a missing transfer annotation

Fix set_image_load_func() introspection.
11 jobs for avatar-gi in 16 minutes and 22 seconds (queued for 2 minutes and 21 seconds)
latest
Status Job ID Name Coverage
  Build
passed #258043
librem5
api-visibility

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passed #258045
librem5
build-debian-clang

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passed #258044
librem5
build-debian-gcc

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passed #258046
librem5
build-fedora-gcc

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  Test+Docs
passed #258047
librem5
build-gtkdoc

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passed #258048
librem5
unit-test

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  Package
passed #258049
librem5
package-debian-buster

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passed #258050
librem5:arm64
package-debian-buster:arm64

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passed #258051
flatpak
package-flatpak

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  Test Package
passed #258052
librem5
autopkgtest-debian-buster-package

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passed #258053
librem5
lintian-debian-buster-package

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