Fix dynamic loading of libsodium

[k-takata]

When dynamic loading is used for libsodium, link error occurs after #11656. Fix it.

[codecov]

Codecov Report

Merging #11667 (5aa57b2) into master (7155fb6) will decrease coverage by 0.00%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master   #11667      +/-   ##
==========================================
- Coverage   81.85%   81.84%   -0.01%     
==========================================
  Files         164      164              
  Lines      190484   190488       +4     
  Branches    43294    43294              
==========================================
- Hits       155914   155907       -7     
- Misses      21934    21953      +19     
+ Partials    12636    12628       -8     

Flag	Coverage Δ
huge-clang-none	82.71% <14.28%> (-0.01%)	⬇️
huge-gcc-none	54.60% <0.00%> (+<0.01%)	⬆️
huge-gcc-testgui	53.05% <0.00%> (+<0.01%)	⬆️
huge-gcc-unittests	0.29% <0.00%> (ø)
linux	82.47% <14.28%> (-0.01%)	⬇️
mingw-x64-HUGE	76.28% <0.00%> (+0.01%)	⬆️
mingw-x86-HUGE	77.07% <0.00%> (+<0.01%)	⬆️
windows	77.92% <0.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/evalfunc.c	90.74% <0.00%> (ø)
src/crypt.c	73.89% <20.00%> (-1.11%)	⬇️
src/ui.c	75.85% <0.00%> (-2.69%)	⬇️
src/os_win32.c	59.70% <0.00%> (-0.11%)	⬇️
src/gui_gtk_x11.c	51.74% <0.00%> (-0.10%)	⬇️
src/mbyte.c	72.70% <0.00%> (-0.07%)	⬇️
src/os_unix.c	67.20% <0.00%> (-0.05%)	⬇️
src/regexp_nfa.c	89.82% <0.00%> (-0.04%)	⬇️
src/regexp_bt.c	86.00% <0.00%> (+0.04%)	⬆️
src/term.c	73.72% <0.00%> (+0.04%)	⬆️
... and 4 more

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