[lmi] Should we update submodules's submodules?

[Greg Chicares]

With gcc-11, I see apparently-harmless warnings (example below)
for what I believe is a libpng submodule embedded in a wx
submodule. This doesn't seem to cause any actual problem, but
perhaps it's worth mentioning.

/opt/lmi/src/lmi/third_party/wx/src/png/png.c: In function 
'wx_png_convert_to_rfc1123_buffer':
/opt/lmi/src/lmi/third_party/wx/src/png/pngprefix.h:136:27: warning: 
'number_buf' may be used uninitialized [-Wmaybe-uninitialized]
  136 | #define png_format_number wx_png_format_number
/opt/lmi/src/lmi/third_party/wx/src/png/png.c:757:69: note: in definition of 
macro 'APPEND_STRING'
  757 | #     define APPEND_STRING(string) pos = png_safecat(out, 29, pos, 
(string))
      |                                                                     
^~~~~~
/opt/lmi/src/lmi/third_party/wx/src/png/pngpriv.h:1758:4: note: in expansion of 
macro 'png_format_number'
 1758 |    png_format_number(buffer, buffer + (sizeof buffer), format, number)
      |    ^~~~~~~~~~~~~~~~~
/opt/lmi/src/lmi/third_party/wx/src/png/png.c:759:24: note: in expansion of 
macro 'PNG_FORMAT_NUMBER'
  759 |          APPEND_STRING(PNG_FORMAT_NUMBER(number_buf, format, (value)))
      |                        ^~~~~~~~~~~~~~~~~