squid: fix compilation due to missing include of time.h

Fixes http://autobuild.buildroot.net/results/b33/b33e1f41e50b6e7ac3e30806b9a617d451bc27b4/



Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>