ppp: avoid strncpy() in ppp plugin nm_phasechange()

strncpy() is deemed insecure, and it raises at least an eyebrow.
While it's save in this case, just avoid it.

(cherry picked from commit 4d11eba8c5)

src/ppp/nm-pppd-plugin.c

@@ -147,7 +147,7 @@ nm_phasechange (void *data, int arg)
 		if (   if_indextoname (index, new_name)
 		    && !nm_streq0 (ifname, new_name)) {
 			g_message ("nm-ppp-plugin: interface name changed from '%s' to '%s'", ifname, new_name);
-			strncpy (ifname, new_name, IF_NAMESIZE);
+			g_strlcpy (ifname, new_name, IF_NAMESIZE);
 		}
 	}
 }