community/shadow: add libcap-dev to enable HAVE_SYS_CAPABILITY_H

Add `libcap-dev` to `makedepends` for enabling `HAVE_SYS_CAPABILITY_H` flag.
https://github.com/shadow-maint/shadow/blob/4.8.1/libmisc/idmapping.c#L150-L185

Required for https://github.com/moby/buildkit/issues/2038

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
8 jobs for !25365 with shadow-uidmap-with-sys-capability-h in 3 minutes and 14 seconds (queued for 1 second)
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Status Job ID Name Coverage
  Lint
passed #487670
docker-alpine x86_64
lint

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  Build
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docker-alpine ci-build aarch64
build-aarch64

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docker-alpine ci-build armhf
build-armhf

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docker-alpine ci-build armv7
build-armv7

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docker-alpine ci-build ppc64le
build-ppc64le

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docker-alpine ci-build s390x
build-s390x

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docker-alpine ci-build x86
build-x86

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docker-alpine ci-build x86_64
build-x86_64

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