arm64: spectre: provide prototypes for internal functions

The helpers in proton-pack.c are called from assembler and have
no prototype in a header, which causes a W=1 warning:

arch/arm64/kernel/proton-pack.c:568:13: error: no previous prototype for 'spectre_v4_patch_fw_mitigation_enable' [-Werror=missing-prototypes]
arch/arm64/kernel/proton-pack.c:588:13: error: no previous prototype for 'smccc_patch_fw_mitigation_conduit' [-Werror=missing-prototypes]
arch/arm64/kernel/proton-pack.c:1064:14: error: no previous prototype for 'spectre_bhb_patch_loop_mitigation_enable' [-Werror=missing-prototypes]
arch/arm64/kernel/proton-pack.c:1075:14: error: no previous prototype for 'spectre_bhb_patch_fw_mitigation_enabled' [-Werror=missing-prototypes]

Add these to asm/spectre.h, which contains related declarations
already.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20230516160642.523862-6-arnd@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>