block: fix linkage problem with blk_iopoll and !CONFIG_BLOCK
block: fix linkage problem with blk_iopoll and !CONFIG_BLOCK

 kernel/built-in.o:(.data+0x17b0): undefined reference to `blk_iopoll_enabled'

Since the extern declaration makes the compile work, but the actual
symbol is missing when block/blk-iopoll.o isn't linked in.

Signed-off-by: Jens Axboe <165ab144a3ccfd9429d5c6466b275f24fafdb114@oracle.com>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs