build: add $(builddir)/$(OS)/$(ARCH) to the headers search list

* Makefile.am (AM_CPPFLAGS): Add -I$(builddir)/$(OS)/$(ARCH) before
-I$(srcdir)/$(OS)/$(ARCH).
diff --git a/Makefile.am b/Makefile.am
index 8241910..cb792ba 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -12,7 +12,10 @@
 
 ACLOCAL_AMFLAGS = -I m4
 AM_CFLAGS = $(WARN_CFLAGS)
-AM_CPPFLAGS = -I$(srcdir)/$(OS)/$(ARCH) -I$(srcdir)/$(OS) -I$(builddir)/$(OS)
+AM_CPPFLAGS = -I$(builddir)/$(OS)/$(ARCH) \
+	      -I$(srcdir)/$(OS)/$(ARCH) \
+	      -I$(builddir)/$(OS) \
+	      -I$(srcdir)/$(OS)
 
 include xlat/Makemodule.am