slub: Fix build breakage in Documentation/vm

This patch fixes a build breakage introduced by commit
f5ac4916e9840292edd33c7a52b10364526547f3 ("slub: move slabinfo.c to
tools/slub/slabinfo.c") that was repoted by Stephen:

  After merging the slab tree, today's linux-next build (x86_64 allmodconfig)
  failed like this:

  gcc: /scratch/sfr/next/Documentation/vm/slabinfo.c: No such file or directory
  gcc: no input files

  Caused by commit f5ac4916e9840292edd33c7a52b10364526547f3 ("slub: move
  slabinfo.c to tools/slub/slabinfo.c").  Missing update to
  Documentation/vm/Makefile?
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

Documentation/vm/Makefile

@@ -2,7 +2,7 @@
 obj- := dummy.o
 
 # List of programs to build
-hostprogs-y := slabinfo page-types hugepage-mmap hugepage-shm map_hugetlb
+hostprogs-y := page-types hugepage-mmap hugepage-shm map_hugetlb
 
 # Tell kbuild to always build the programs
 always := $(hostprogs-y)