sys_linux: fix building with older compilers and some archs

The recent replacement of <termios.h> with <linux/termios.h> to get
TCGETS2 seems to work only with compilers (or C standards) that allow
the same structure to be defined multiple times. There is a conflict
between <sys/ioctl.h> and <linux/termios.h>.

Another problem is that TCGETS2 is not used on some archs like ppc64.

Switch back to <termios.h> and move TCGETS2 to a list in a separate
file where it can be compiled without <sys/ioctl.h>.

Fixes: 03875f1ea5 ("sys_linux: allow ioctl(TCGETS2) in seccomp filter")

sys_linux_scmp.h

@@ -0,0 +1,28 @@
+/*
+  chronyd/chronyc - Programs for keeping computer clocks accurate.
+
+ **********************************************************************
+ * Copyright (C) Miroslav Lichvar  2025
+ * 
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ * 
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ * 
+ **********************************************************************
+
+  =======================================================================
+
+  Header file for lists that are needed in seccomp filters but need to
+  be compiled separately from sys_linux.c due to conflicting headers.
+  */
+
+extern unsigned long SYS_Linux_GetExtraScmpIoctl(int index);