pkcs15-tool: Build with current gcc

The argument to strncpy is not the length of the target buffer,
but the source one (excluding the null byte, which will be
copied anyway).

src/tools/pkcs15-tool.c

@@ -1022,7 +1022,7 @@ static int read_ssh_key(void)
 		buf[1] = 0;
 		buf[2] = 0;
 		len = snprintf((char *) buf+4, 20, "ecdsa-sha2-nistp%d", n);
-		strncpy(alg, (char *) buf+4, 20);
+		strncpy(alg, (char *) buf+4, 19);
 		buf[3] = len;
 
 		len += 4;