powerpc64: fix build regression

* linux/powerpc64/arch_regs.h: New file.
* Makefile.am (EXTRA_DIST): Add it.
diff --git a/Makefile.am b/Makefile.am
index b83b6fc..86ea59b 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -253,6 +253,7 @@
 	linux/powerpc/ioctls_inc0.h	\
 	linux/powerpc/syscallent.h	\
 	linux/powerpc/userent.h		\
+	linux/powerpc64/arch_regs.h	\
 	linux/powerpc64/errnoent.h	\
 	linux/powerpc64/errnoent1.h	\
 	linux/powerpc64/ioctls_arch0.h	\