pcr: aflplusplus: Add clang build dependency
    
    Without that fix we have:
        [+] llvm_mode detected llvm 11+, enabling afl-lto LTO implementation
        GNUmakefile.llvm:120: we have trouble finding clang - llvm-config is not helping us
        GNUmakefile.llvm:135: we have trouble finding clang++ - llvm-config is not helping us
        GNUmakefile.llvm:223: clang option -flto is not working - maybe LLVMgold.so not found - cannot enable LTO mode
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>