core: Include cstring whenever strncpy is used (#7267)

Fixes ppc64le build in alpine

src/helpers/SdDaemon.cpp

@@ -8,6 +8,7 @@
 #include <sys/socket.h>
 #include <sys/un.h>
 #include <string.h>
+#include <cstring>
 
 namespace Systemd {
     int SdBooted(void) {