scripts: setlocalversion: replace backquote to dollar parenthesis

This patch replaces backquote to dollar parenthesis syntax for better
readability.

Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Nico Schottelius <nico-linuxsetlocalversion@schottelius.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>