shared: introduce timespec-util.h

Copyright is set according to the moved code from compositor.c.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/Makefile.am b/Makefile.am
index a0d0c19..eb3e867 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -90,6 +90,7 @@
 	shared/helpers.h				\
 	shared/matrix.c					\
 	shared/matrix.h					\
+	shared/timespec-util.h				\
 	shared/zalloc.h					\
 	shared/platform.h				\
 	src/weston-egl-ext.h