pcr: apiguardian: fix missing pom.xml
    
    While apiguardian uses Maven, there is no pom.xml but instead a
    build.gradle.kts file.
    
    I'm not sure if replacing the pom.xml with the build.gradle.kts is the
    right fix, but before it did complain:
        |  ==> Starting package()...
        |  install: cannot stat 'pom.xml': No such file or directory
        |  ==> Tidying install...
    and now it doesn't complain anymore.
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>