build: fix seq host tool check

Problem found on alpine linux when trying to `./scripts/feeds update -a`,
which results in `Build dependency: Missing seq command`.

Ref: openwrt/openwrt#1926
Suggested-by: imShara <shara@protonmail.com>
[reworded commit and turned faulty Sob into Suggested-by]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

include/prereq-build.mk

@@ -105,9 +105,9 @@ $(eval $(call SetupHostCommand,cp,Please install GNU fileutils, \
 	gcp --help 2>&1 | grep 'Copy SOURCE', \
 	cp --help 2>&1 | grep 'Copy SOURCE'))
 
-$(eval $(call SetupHostCommand,seq,, \
+$(eval $(call SetupHostCommand,seq,Please install seq, \
 	gseq --version, \
-	seq --version))
+	seq --version 2>&1 | grep seq))
 
 $(eval $(call SetupHostCommand,awk,Please install GNU 'awk', \
 	gawk --version 2>&1 | grep GNU, \