configure: Remove use of backtick `...` syntax

There's only one place in configure where we use `...` to execute a
command and capture the result.  Switch to $() to match the rest of
the script. This silences a shellcheck warning.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20220825150703.4074125-6-peter.maydell@linaro.org

configure

@@ -2311,7 +2311,7 @@ LINKS="$LINKS python"
 LINKS="$LINKS contrib/plugins/Makefile "
 for f in $LINKS ; do
     if [ -e "$source_path/$f" ]; then
-        mkdir -p `dirname ./$f`
+        mkdir -p "$(dirname ./"$f")"
         symlink "$source_path/$f" "$f"
     fi
 done