DocBook: Do not exceed argument list limit

Use find + xargs to compress the generated manpages. Without this patch,
the build can fail with

    gzip -f Documentation/DocBook/man/*.9
    /bin/bash: /usr/bin/gzip: Argument list too long

This happened with qemu user mode emulation on aarch64.

Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/DocBook/Makefile

@@ -56,7 +56,7 @@ htmldocs: $(HTML)
 
 MAN := $(patsubst %.xml, %.9, $(BOOKS))
 mandocs: $(MAN)
-	$(if $(wildcard $(obj)/man/*.9),gzip -f $(obj)/man/*.9)
+	find $(obj)/man -name '*.9' | xargs gzip -f
 
 installmandocs: mandocs
 	mkdir -p /usr/local/man/man9/