prereq-build: flex is built in tools/ - do not require it to be installed on the host

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@32116 3c298f89-4303-0410-b956-a3cf2f4a3e73

include/prereq-build.mk

@@ -93,10 +93,6 @@ $(eval $(call RequireCommand,gawk, \
 	Please install GNU awk. \
 ))
 
-$(eval $(call RequireCommand,flex, \
-	Please install flex. \
-))
-
 $(eval $(call RequireCommand,unzip, \
 	Please install unzip. \
 ))