cpu/x86/lapic: Declare start_cpu() static

This is for the !PARALLEL_MP paths.

Change-Id: If4b91834a1b6de2a902ab914610ab76c1423f1e9
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55188
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/cpu/x86/lapic/lapic_cpu_init.c

@@ -241,7 +241,7 @@ static void *stacks[CONFIG_MAX_CPUS];
 volatile unsigned long secondary_stack;
 volatile unsigned int secondary_cpu_index;
 
-int start_cpu(struct device *cpu)
+static int start_cpu(struct device *cpu)
 {
 	struct cpu_info *info;
 	uintptr_t stack_top;