scripts: gcc-wrapper: Add extents.c to allowed warning list

With gcc-4.5.3 compiler, we get the error:
In function 'ext4_ext_map_blocks':
kernel/fs/ext4/extents.c:2091:24:
warning: 'cex.ec_start' may be used uninitialized in this function

Change-Id: I460abf305a725deb3c3b06582518f8cd35314385
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>