build-sys: invoke ninja with -d keepdepfile

After reading the dependency file, ninja just deletes it, in the name
of cleanliness I guess.  However this complicates debugging unnecessarily
compared to good old "-include *.d".  Use the keepdepfile debugging
option to make it easier to see what is going on.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210215122103.63933-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Makefile

@@ -149,7 +149,7 @@ $(ninja-targets): run-ninja
 # --output-sync line.
 run-ninja: config-host.mak
 ifneq ($(filter $(ninja-targets), $(ninja-cmd-goals)),)
-	+$(quiet-@)$(if $(MAKE.nq),@:, $(NINJA) \
+	+$(quiet-@)$(if $(MAKE.nq),@:, $(NINJA) -d keepdepfile \
 	   $(NINJAFLAGS) $(sort $(filter $(ninja-targets), $(ninja-cmd-goals))) | cat)
 endif
 endif