openblocks_a6_defconfig: bump to linux-4.11.5 to fix build with gcc 6

openblocks_a6_defconfig: bump to linux-4.11.5 to fix build with gcc 6

The recent change to default to gcc 6 for the internal toolchain broke this
defconfig, as the kernel doesn't contain commit cb984d101b (compiler-gcc:
integrate the various compiler-gcc[345].h files) which was added during the
4.2 cycle.

Fix the build by bumping the kernel to 4.11.5.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>