community/mlt: fix deadlock due to recursive mutex

See: alpine/aports#11838 (comment 114554)

cherry picked from commit 9558f935
7 jobs for !12640 with mlt-fix-mutex-3.12-stable in 3 minutes and 49 seconds (queued for 1 second)
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Status Job ID Name Coverage
  Lint
failed #210367
docker-alpine x86_64 allowed to fail
lint

00:00:20

 
  Build
passed #210372
docker-alpine ci-build aarch64
build-aarch64

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passed #210373
docker-alpine ci-build armv7
build-armv7

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passed #210371
docker-alpine ci-build ppc64le
build-ppc64le

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passed #210370
docker-alpine ci-build s390x
build-s390x

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passed #210369
docker-alpine ci-build x86
build-x86

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passed #210368
docker-alpine ci-build x86_64
build-x86_64

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failed
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