Disable stringop-truncation false positive warnings on gcc 8.

Strings in struct utmp/utmpx are not guaranteed to be NUL-terminated.

src/utmp.c

@@ -52,6 +52,14 @@
 #include "sudo.h"
 #include "sudo_exec.h"
 
+/*
+ * GCC 8 warns about strncpy() where the size field is the size of the buffer.
+ * However, strings in utmp may not be NUL terminated so this usage is correct.
+ */
+#if __GNUC_PREREQ__(8, 0)
+# pragma GCC diagnostic ignored "-Wstringop-truncation"
+#endif
+
 /*
  * Simplify handling of different utmp types.
  */