libselinux: fix build order

We need to install the include files before we try to build the source.
Otherwise, make DESTDIR=~/obj install can fail if there are older
headers under /usr/include.

Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
diff --git a/libselinux/Makefile b/libselinux/Makefile
index 16531fe..6a43b24 100644
--- a/libselinux/Makefile
+++ b/libselinux/Makefile
@@ -1,4 +1,4 @@
-SUBDIRS = src include utils man
+SUBDIRS = include src utils man
 
 PKG_CONFIG ?= pkg-config
 DISABLE_SETRANS ?= n