macos-setup: fix a test.

The syntax

    if [ <test> ]; then
        ...
    fi

isn't what one might think.  The way that works is that "[" is a
command; it's an alias for "test", except that if it's involked as "["
rather than as "test", it expects there to be a token "]" at the end, so
that the test expression is enclosed in square brackets.  (This dates
back all the way to, I think, V7, although the link from "/bin/test" to
"/bin/[" wasn't documented at that point.)

This means that the "]" must have white space before it, so it's a
separate token.

[skip ci]

tools/macos-setup.sh

@@ -1615,7 +1615,7 @@ uninstall_snappy() {
         # just remove what we know it installs.
         #
         # $DO_MAKE_UNINSTALL || exit 1
-        if [ -s build_dir/install_manifest.txt] ; then
+        if [ -s build_dir/install_manifest.txt ] ; then
             while read -r ; do $DO_RM -v "$REPLY" ; done < <(cat build_dir/install_manifest.txt; echo)
         else
             $DO_RM -f /usr/local/lib/libsnappy.1.1.8.dylib \