Add shellcheck to pre-commit

Closes #22

privatize_symbols_macos.sh

@@ -2,5 +2,5 @@
 
 # This has the effect of making visibility=hidden symbols private in object files
 for obj in "$@" ; do
-  ld -r $obj -o $obj.tmp && mv $obj.tmp $obj
+  ld -r "$obj" -o "$obj.tmp" && mv "$obj.tmp" "$obj"
 done