Fix build errors in dependent packages if PREFIX is different from ECM's
PREFIX - which is the case in Guix. See <https://bugs.kde.org/424483> for
details.
* gnu/packages/patches/plasma-framework-fix-KF5PackageMacros.cmake.patch: New
file.
* gnu/local.mk (dist_patch_DATA) Add it.
* packages/kde-frameworks.scm (package)[source]: Use this patch.
