New required MPFR dependency for kcalc 19.08

[Christoph Feck]

Hi,

I have just committed https://phabricator.kde.org/D21495 for kcalc.
It adds a required dependency to MPFR library for the upcoming 19.08
releases.

Unfortunately I didn't expect the CI to not have this dependency, so
right now it fails to build for openSUSE (but works for FreeBSD).

https://build.kde.org/job/Applications/job/kcalc/

Sorry for not checking earlier.

-- 
Christoph Feck
KDE Release Team