asm-generic/tlb: add missing CONFIG symbol

asm-generic/tlb: add missing CONFIG symbol

Without this the symbol will not actually end up in .config files.

Link: http://lkml.kernel.org/r/20200116064531.483522-6-aneesh.kumar@linux.ibm.com
Fixes: 37bd8fe1b8d6 ("asm-generic/tlb: Provide generic tlb_flush() based on flush_tlb_mm()")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>