Add missing include for `sys/stat.h`.

Building using `brew install znc --HEAD` resulted in the following:

    Building module notify_connect...
    brew: superenv removed: -I../include -O2 -Wall -W -Wno-unused-parameter -Woverloaded-virtual -Wshadow
    log.cpp:100:14: error: variable has incomplete type 'struct stat'
            struct stat ModDirInfo;
                        ^
    ./../include/znc/FileUtils.h:93:50: note: forward declaration of 'stat'
            static int GetInfo(const CString& sFile, struct stat& st);