kasan: fix a missing header include of static_keys.h

kasan: fix a missing header include of static_keys.h

The kasan-enabled.h header relies on static keys, so make sure
to include the header to avoid compilation errors (with JUMP_LABEL=n).

It fixes the following:
./include/linux/kasan-enabled.h:9:1: warning: data definition has no type or storage class
    9 | DECLARE_STATIC_KEY_FALSE(kasan_flag_enabled);
      | ^~~~~~~~~~~~~~~~~~~~~~~~
error: type defaults to 'int' in declaration of 'DECLARE_STATIC_KEY_FALSE' [-Werror=implicit-int]

Fixes: f9b5e46f4097eb29 ("kasan: split kasan_*enabled() functions into a separate header")
Cc: Peter Collingbourne <pcc@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Acked-by: Andrey Konovalov <andreyknvl@gmail.com>
Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20220301154518.19456-1-joey.gouly@arm.com
Signed-off-by: Will Deacon <will@kernel.org>