Makefile: add missing mkdir MANUAL_BUILDDIR

The MANUAL_BUILDDIR directory is automatically created by sphinx-build
for the other targets.  The index.html target does not use sphinx-build
so we must manually create the directory to avoid the following error:

  GEN     docs/built/index.html
  /bin/sh: docs/built/index.html: No such file or directory

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20200120163400.603449-1-stefanha@redhat.com
Reviewed-by: Miroslav Rezanina <mrezanin@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

Makefile

@@ -1022,6 +1022,7 @@ $(MANUAL_BUILDDIR)/interop/qemu-ga.8: $(call manual-deps,interop)
 	$(call build-manual,interop,man)
 
 $(MANUAL_BUILDDIR)/index.html: $(SRC_PATH)/docs/index.html.in qemu-version.h
+	@mkdir -p "$(MANUAL_BUILDDIR)"
 	$(call quiet-command, sed "s|@@VERSION@@|${VERSION}|g" $< >$@, \
              "GEN","$@")