disk-image-minimal: Prepare for libsys's existence

pycheribuild/projects/disk_image.py

@@ -1056,6 +1056,8 @@ def add_required_libraries(self, libdirs: "list[str]", ignore_required: bool = F
  "libz.so.6",
  "librt.so.1",
  ]
+ # required, but only recently added so not always present
+ optional_libs += ["libsys.so.7"]
  # required, but versions were bumped with changes to ncurses
  optional_libs += [
  # needed by /bin/sh & /bin/csh (if we included the purecap sh/csh)