Add a patch to fix the build with newer versions of SIP.

We're currently testing an update to SIP 4.19.6, but it breaks qtiplot's build.

This patch, obtained via Debian and Gentoo, allows the port to build with both
SIP 4.19.2 (currently in the tree) as well as 4.19.6.

In the future, we might look into disabling Python scripting support: it tends
to break with almost every SIP update, and upstream has stopped releasing the
source code for newer qtiplot releases.

PR: 224739
Obtained from: https://anonscm.debian.org/git/debian-science/packages/qtiplot.git/tree/debian/patches/20_fix_FTBFS_sip4.19.patch
Obtained from: https://bugs.gentoo.org/641514#c2