configure: change grep -P to grep -E

grep -P is not supported on FreeBSD. So use grep -E when we check
whether the value provided for --seed-buckets is a number or not.

Reported-by: Rebecca Cran <rebecca@bsdio.com>
Link: https://lore.kernel.org/fio/26076f36-54d5-ca6e-0b96-a7371b1c5e49@bsdio.com/T/#t
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>

configure

@@ -117,7 +117,7 @@ has() {
 }
 
 num() {
-  echo "$1" | grep -P -q "^[0-9]+$"
+  echo "$1" | grep -E -q "^[0-9]+$"
 }
 
 check_define() {