Ignore failure at moving revision.h [ci skip]

The source directory may be read-only.

common.mk

@@ -1234,7 +1234,7 @@ revision.$(HAVE_BASERUBY:yes=tmp):: $(srcdir)/version.h $(tooldir)/file2lastrev.
 	$(Q) $(BASERUBY) $(tooldir)/file2lastrev.rb -q --revision.h --srcdir="$(srcdir)" > $@
 
 $(REVISION_H): revision.tmp
-	$(Q)$(IFCHANGE) "--timestamp=$@" "$(srcdir)/revision.h" revision.tmp
+	-$(Q)$(IFCHANGE) "--timestamp=$@" "$(srcdir)/revision.h" revision.tmp
 
 $(srcdir)/ext/ripper/ripper.c: $(srcdir)/ext/ripper/tools/preproc.rb $(srcdir)/parse.y $(srcdir)/defs/id.def $(srcdir)/ext/ripper/depend
 	$(ECHO) generating $@