slack-desktop: add -p to mkdir to avoid error

[acobaugh]

I encountered this error when attempting to build slack-desktop locally. Unsure if there's a better fix, but it seems harmless.

=> slack-desktop-4.12.0_1: running do_install ...
mkdir: cannot create directory ‘/destdir//slack-desktop-4.12.0/usr/lib’: File exists
=> ERROR: slack-desktop-4.12.0_1: do_install: 'mkdir ${DESTDIR}/usr/{bin,lib}' exited with 1
=> ERROR:   in do_install() at srcpkgs/slack-desktop/template:27

[Chocimier]

Thanks for contribution.

Please remove creation of usr/lib instead. It is created by xbps-src now.
Would be great if you could do the same in textadept.

[acobaugh]

That makes more sense as to why this was suddenly breaking for me. Pushed the changes.