https://cgit.freedesktop.org/xorg/xserver/commit/?id=30ac7567980a1eb79d084a63e0e74e1d9a3af673

this part of code  breaks build of VBox 

diff --git a/include/os.h b/include/os.h
index e9b3709..20224f1 100644
--- a/include/os.h
+++ b/include/os.h
@@ -706,4 +706,9 @@ xorg_backtrace(void);
 extern _X_EXPORT int
 os_move_fd(int fd);
 
+#include <signal.h>
+
+extern _X_EXPORT int
+xthread_sigmask(int how, const sigset_t *set, sigset_t *oldest);
+
 #endif                          /* OS_H */


Builds ends with:
In file included from /usr/include/xorg/misc.h:117:0,
                 from /usr/include/xorg/xf86str.h:37,
                 from /builddir/build/BUILD/VirtualBox-5.1.6/src/VBox/Additions/x11/vboxvideo/vboxvideo.h:107,
                 from /builddir/build/BUILD/VirtualBox-5.1.6/src/VBox/Additions/x11/vboxvideo/getmode.c:18:
/usr/include/xorg/os.h:708:32: error: unknown type name 'sigset_t'
 xthread_sigmask(int how, const sigset_t *set, sigset_t *oldest);
                                ^~~~~~~~
/usr/include/xorg/os.h:708:47: error: unknown type name 'sigset_t'
 xthread_sigmask(int how, const sigset_t *set, sigset_t *oldest);


where should be defined sigset_t ? 
removing these 3 lines Vbox builds without problems , what I am missing ? 

Thanks,