scripts/strip-kmod.sh: replace legacy backticks with $()

This replaces deprecated backticks by more versatile $(...) syntax.

Found with shellcheck.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
[slightly adjusted commit title, added commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

scripts/strip-kmod.sh

@@ -50,6 +50,6 @@ $3 && $2 ~ /[brtd]/ && $3 !~ /\$LC/ && !def[$3] {
 }
 ' > "$MODULE.tmp1"
 
-${CROSS}objcopy `cat ${MODULE}.tmp1` ${MODULE}.tmp ${MODULE}.out
+${CROSS}objcopy $(cat ${MODULE}.tmp1) ${MODULE}.tmp ${MODULE}.out
 mv "${MODULE}.out" "${MODULE}"
 rm -f "${MODULE}".t*