util/qemu-openpty: fix build with musl libc by include termios.h as fallback

Include termios.h as POSIX fallback when not glibc, bsd or solaris.
POSIX says that termios.h should define struct termios and TCAFLUSH.
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/termios.h.html

This fixes the following compile errors with musl libc:

util/qemu-openpty.c: In function 'qemu_openpty_raw':
util/qemu-openpty.c:112:20: error: storage size of 'tty' isn't known
     struct termios tty;
                    ^
...
util/qemu-openpty.c:128:24: error: 'TCSAFLUSH' undeclared (first use in this function)
     tcsetattr(*aslave, TCSAFLUSH, &tty);
                        ^

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

util/qemu-openpty.c

@@ -47,6 +47,8 @@
 #elif defined CONFIG_SOLARIS
 # include <termios.h>
 # include <stropts.h>
+#else
+# include <termios.h>
 #endif
 
 #ifdef __sun__