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 <[email protected]>

target/linux/generic/config-4.14

@@ -2678,6 +2678,7 @@ CONFIG_MISC_FILESYSTEMS=y
 # CONFIG_MISDN_NETJET is not set
 # CONFIG_MISDN_SPEEDFAX is not set
 # CONFIG_MISDN_W6692 is not set
+CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY=y
 # CONFIG_MKISS is not set
 # CONFIG_MLX4_CORE is not set
 # CONFIG_MLX4_EN is not set