icu: propagate host gcc dependency to reverse dependencies

Fixes:
http://autobuild.buildroot.net/results/821/821eae7add252e3711ed069a2d292a04427fa43b/



Commit 87f43c34 (package/icu: needs host gcc >= 4.8) added a new toolchain
dependency to icu, but didn't propagate it to the reverse dependencies of
icu.

Some of these have been fixed in later commits, but not all.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>