scripts: gcc-wrapper: Add an allowed warning for block.c

Commit f2b3f49ede3be96e11b60880c22a768328ce4d2f added a warning
from Open Emdedded's gcc-4.6.3 compiler.

| kernel/drivers/mmc/card/block.c: In function 'mmc_blk_issue_rq':
| kernel/drivers/mmc/card/block.c:836:18:
| warning: 'arg' may be used uninitialized in this function
| [-Wuninitialized] error, forbidden warning: block.c:836
| kernel/drivers/mmc/card/block.c:835:6: warning: 'nr' may be used
| uninitialized in this function [-Wuninitialized]
| error, forbidden warning: block.c:835

Change-Id: I05a45033e5dae8b126cbbba89188d84244485afd
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>
(cherry picked from commit 3d2c28943e880d6a0e7619c4234d8a52ccf8f9e1)