scripts: gcc-wrapper: Allow few forbidden warnings

Enabling PPPOPNS config will enable pppopns.c file
for compilation.Bypassing forbidden warnings in
pppopns by adding them to a whitelist in gcc-wrapper.

Change-Id: If2b3e76b4b467339d18d2cfe1c90828434e1ba07
Signed-off-by: Ravinder Konka <rkonka@codeaurora.org>