[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.
Merge pull request #18 from amtep/master

[msyncd] let plugins see msyncd's symbols