soc/mtk_adsp: Add missing z_prep_c() prototype

The early boot function got renamed to a pseudo-standard "z_prep_c",
but this isn't an actual API and doesn't have a prototype in the
headers anywhere, so the compiler started whining about an undeclared
function.

Signed-off-by: Andy Ross <andyross@google.com>

soc/mediatek/mt8xxx/soc.c

@@ -179,5 +179,6 @@ void c_boot(void)
 	/* Default console, a driver can override this later */
 	__stdout_hook_install(arch_printk_char_out);
 
+	void z_prep_c(void);
 	z_prep_c();
 }