Move definition of struct xlat to a separate header file

Define struct xlat in a separate file so that it could be used later by
tests without inclusion of defs.h header file.

* defs.h (struct xlat, XLAT, XLAT_END): Move ...
* xlat.h: ... here.
* Makefile.am (strace_SOURCES): Add xlat.h.
diff --git a/Makefile.am b/Makefile.am
index 2f595cb..f02a421 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -214,6 +214,7 @@
 	vsprintf.c	\
 	wait.c		\
 	xattr.c		\
+	xlat.h		\
 	xmalloc.c	\
 	# end of strace_SOURCES