kill warnings when building mandocs

This patch shuts warnings of the sort:

make -C /mnt/samsung_200/sam/kernel/trees/21-rc6/build \
	KBUILD_SRC=/mnt/samsung_200/sam/kernel/trees/21-rc6 \
	KBUILD_EXTMOD="" -f /mnt/samsung_200/sam/kernel/trees/21-rc6/Makefile mandocs
make -f /mnt/samsung_200/sam/kernel/trees/21-rc6/scripts/Makefile.build obj=scripts/basic
make -f /mnt/samsung_200/sam/kernel/trees/21-rc6/scripts/Makefile.build obj=Documentation/DocBook mandocs
  SRCTREE=/mnt/samsung_200/sam/kernel/trees/21-rc6/ /mnt/samsung_200/sam/kernel/trees/21-rc6/build/scripts/basic/docproc doc /mnt/samsung_200/sam/kernel/trees/21-rc6/Documentation/DocBook/wanbook.tmpl >Documentation/DocBook/wanbook.xml
  if grep -q refentry Documentation/DocBook/wanbook.xml; then xmlto man -m /mnt/samsung_200/sam/kernel/trees/21-rc6/Documentation/DocBook/stylesheet.xsl -o Documentation/DocBook/man Documentation/DocBook/wanbook.xml ; gzip -f Documentation/DocBook/man/*.9; fi
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