add missing pkgconf build dependency in recent libxslt easyconfigs

[Micket]

(created using eb --new-pr)
Fixes #17202

[Micket]

Test report by @Micket
SUCCESS
Build succeeded for 10 out of 10 (6 easyconfigs in total)
alvis-c1 - Linux Rocky Linux 8.6, x86_64, Intel Xeon Processor (Skylake), Python 3.6.8
See https://gist.github.com/dd8aef654db38ddc2d1b08c46491253b for a full test report.

[branfosj]

Test report by @branfosj
SUCCESS
Build succeeded for 9 out of 9 (7 easyconfigs in total)
bear-pg0211u03a.bear.cluster - Linux RHEL 8.6, x86_64, Intel(R) Xeon(R) Gold 6248 CPU @ 2.50GHz (cascadelake), Python 3.6.8
See https://gist.github.com/a77963be1305a0fc81286c5f71f4d5b4 for a full test report.

[branfosj]

@boegelbot please test @ generoso

[boegelbot]

@branfosj: Request for testing this PR well received on login1

PR test command 'EB_PR=17254 EB_ARGS= EB_CONTAINER= /opt/software/slurm/bin/sbatch --job-name test_PR_17254 --ntasks=4 ~/boegelbot/eb_from_pr_upload_generoso.sh' executed!

• exit code: 0
• output:

Submitted batch job 10113

Test results coming soon (I hope)...

- notification for comment with ID 1416723847 processed

Message to humans: this is just bookkeeping information for me,
it is of no use to you (unless you think I have a bug, which I don't).

[boegelbot]

Test report by @boegelbot
SUCCESS
Build succeeded for 7 out of 7 (7 easyconfigs in total)
cns1 - Linux Rocky Linux 8.5, x86_64, Intel(R) Xeon(R) CPU E5-2667 v3 @ 3.20GHz (haswell), Python 3.6.8
See https://gist.github.com/d6683d8195e51fe9edda49fc8b734a48 for a full test report.

[branfosj]

@boegelbot please test @ jsc-zen2

[boegelbot]

@branfosj: Request for testing this PR well received on jsczen2l1.int.jsc-zen2.easybuild-test.cluster

PR test command 'EB_PR=17254 EB_ARGS= /opt/software/slurm/bin/sbatch --mem-per-cpu=4000M --job-name test_PR_17254 --ntasks=8 ~/boegelbot/eb_from_pr_upload_jsc-zen2.sh' executed!

• exit code: 0
• output:

Submitted batch job 2156

Test results coming soon (I hope)...

- notification for comment with ID 1416725459 processed

Message to humans: this is just bookkeeping information for me,
it is of no use to you (unless you think I have a bug, which I don't).

[boegelbot]

Test report by @boegelbot
SUCCESS
Build succeeded for 7 out of 7 (7 easyconfigs in total)
jsczen2c1.int.jsc-zen2.easybuild-test.cluster - Linux Rocky Linux 8.5, x86_64, AMD EPYC 7742 64-Core Processor (zen2), Python 3.6.8
See https://gist.github.com/9695f8ded357dfb0d07da029cb322cb7 for a full test report.

[branfosj]

Going in, thanks @Micket!