Include <iterator>.

In order to fix:
[ 16s] /home/abuild/rpmbuild/BUILD/libkleo-21.12.3/src/kleo/keyserverconfig.cpp:73:44: required from here
[ 16s] /home/abuild/rpmbuild/BUILD/libkleo-21.12.3/src/utils/algorithm.h:35:30: error: no matching function for call to 'begin(QStringList&)'
[ 16s] 35 | std::transform(std::begin(c), std::end(c), std::begin(c), op);
[ 16s] | ~~~~~~~~~~^~~

The change is documented here:
https://gcc.gnu.org/gcc-12/porting_to.html#header-dep-changes