Draft: Fix #if vs. #ifdef
This broke Craft builds as you can see here: https://binary-factory.kde.org/view/Management/job/Craft_Build_Master_Cache_linux-64-gcc/39/execution/node/18/log/
/CMakeFiles/KF5WaylandClient.dir/shm_pool.cpp.o.d -o src/client/CMakeFiles/KF5WaylandClient.dir/shm_pool.cpp.o -c /home/appimage/Craft/BC/linux-64-gcc/build/kde/frameworks/tier1/kwayland/work/kwayland-5.101.0/src/client/shm_pool.cpp
/home/appimage/Craft/BC/linux-64-gcc/build/kde/frameworks/tier1/kwayland/work/kwayland-5.101.0/src/client/shm_pool.cpp:119:15: error: #if with no expression
119 | #if HAVE_MEMFD
| ^