eglibc: do not prompt for the revision, it is supposed to be changed by developers, not users.

fixes changing the eglibc version in menuconfig without resetting the config

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@34029 3c298f89-4303-0410-b956-a3cf2f4a3e73