scripts: gcc-wrapper: Allow warnings from google-common

3rd party Google-common code contains the following forbidden warning when
compiling. Once the remote corrects this warning, this change should be
removed.

ping.c:87:27: warning: assignment from incompatible pointer type

Change-Id: I278bc583884576ded13585f35f8be5ab9b2c5eef
Signed-off-by: Ian Maund <imaund@codeaurora.org>