community/flatpak-builder: add elfutils dependency

This is required for building most C/CPP packages (e.g. xournalpp), which
need eu-strip
7 jobs for !1639 with flatpak-builder in 3 minutes and 22 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Lint
passed #17530
x86_64 docker-alpine allowed to fail
lint

00:00:20

 
  Build
passed #17535
aarch64 ci-build docker-alpine
build-aarch64

00:01:13

passed #17536
armv7 ci-build docker-alpine
build-armv7

00:01:16

passed #17534
ppc64le ci-build docker-alpine
build-ppc64le

00:01:01

passed #17533
s390x ci-build docker-alpine
build-s390x

00:00:37

passed #17532
x86 ci-build docker-alpine
build-x86

00:00:49

passed #17531
x86_64 ci-build docker-alpine
build-x86_64

00:00:50