Fix reserved register keyword usage for >=cpp17
This will probably also fix freebsd build problem
10:39:45 In file included from /usr/home/jenkins/workspace/Extragear/digikam/kf5-qt5 FreeBSDQt5.15/core/libs/dimg/filters/icc/iccprofile.cpp:27:
10:39:45 In file included from /usr/home/jenkins/workspace/Extragear/digikam/kf5-qt5 FreeBSDQt5.15/core/libs/dimg/filters/icc/digikam-lcms.h:52:
10:39:45 /usr/local/include/lcms2.h:1261:44: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]