mtree: Keep mtree files indented with spaces

The mtree tool indents directory entries with 4 spaces.

Reviewed by:	imp
Fixes:		9cab9fde5e ("virtual_oss: Port to base")
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D53979

etc/mtree/BSD.lib32.dist

@@ -21,7 +21,7 @@
         ..
         pkgconfig
         ..
-	virtual_oss
-	..
+        virtual_oss
+        ..
     ..
 ..

etc/mtree/BSD.usr.dist

@@ -103,8 +103,8 @@
         ..
         ossl-modules
         ..
-	virtual_oss
-	..
+        virtual_oss
+        ..
     ..
     libdata
         ldscripts