Documentation: make Makefile.sphinx no-ops quieter

Silence the "make[1]: Nothing to be done for ..." messages for the
no-op targets in Makefile.sphinx.
Signed-off-by: NJim Davis <jim.epost@gmail.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/Makefile.sphinx

@@ -98,9 +98,13 @@ endif # HAVE_SPHINX
 
 # no-ops for the Sphinx toolchain
 sgmldocs:
+	@:
 psdocs:
+	@:
 mandocs:
+	@:
 installmandocs:
+	@:
 
 cleandocs:
 	$(Q)rm -rf $(BUILDDIR)