ntp: bump to version 4.2.6p3 and fix glibc build failure

Bump to version 4.2.6p3 and fix a build failure with external toolchains
that use certain glibc versions as point out by speakman on IRC.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: Daniel Nyström <daniel.nystrom@timeterminal.se>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>