Fix "make dist" regression introduced by commit v4.7-184-gd648f29

* Makefile.am (EXTRA_DIST): Add linux/ubi-user.h.
diff --git a/Makefile.am b/Makefile.am
index f3ae0ba..71d463d 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -155,6 +155,7 @@
 	linux/syscall.h			\
 	linux/tile/ioctlent.h.in	\
 	linux/tile/syscallent.h		\
+	linux/ubi-user.h		\
 	linux/x32/errnoent.h		\
 	linux/x32/errnoent1.h		\
 	linux/x32/ioctlent.h.in		\