[GN] Add -Wno-unused-result on Linux

fread is used in putil.cpp without examining the return value. Suppress
this warning for ICU code only.

This is done for chromium/m49 branch (that can be used for head, too)
because origin/master is already ICU 56.1 that does not need this
warning to be suppressed.

Original Patch by slan@ : https://codereview.chromium.org/1704763002/
BUG=587965
R=slan@chromium.org

Review URL: https://codereview.chromium.org/1719453003 .
1 file changed