kernel: generic: add missing symbol for arm64 spectre mitigation

kernel: generic: add missing symbol for arm64 spectre mitigation

Upstream in commit 3e3904125fcc ("arm64: Mitigate spectre style branch
history side channels") introduced new config symbol
MITIGATE_SPECTRE_BRANCH_HISTORY which I missed in commit d39a6c67dcb4
("kernel: bump 4.14 to 4.14.275") and buildworkers for arm64 targets
started complaining:

 Mitigate Spectre style attacks against branch history (MITIGATE_SPECTRE_BRANCH_HISTORY) [Y/n/?] (NEW) aborted!

Fixes: d39a6c67dcb4 ("kernel: bump 4.14 to 4.14.275")
Signed-off-by: Petr Štetiar <ynezz@true.cz>