Fix build with newer gcc versions.

Add upstream patch [1] that uses

• isnan() -> std::isnan()
• isinf() -> std::isinf()

This fixes compilation with gcc5.

[1] https://quickgit.kde.org/?p=kmplot.git&a=commit&h=e903ca56ad26311fcaeb650acf96046c726fb809

PR: 213993
Reported by: Kenneth Salerno <kennethsalerno@yahoo.com>
Approved by: rakuco (mentor)
Obtained from: https://quickgit.kde.org/?p=kmplot.git&a=commit&h=e903ca56ad26311fcaeb650acf96046c726fb809