[msyncd] let plugins see msyncd's symbols

After the change to using invoker to launch msyncd, the fsstorage plugin could no longer find its symbols. Adding -G (--global-syms) to the invoker makes symbol resolution behave the same way as when msyncd is the main program.

This fixes 2 symptoms:
  - an assertion failure from msyncd when taking the USB cable out
  - inability to see any files from the host (due to fsstorage not loading)