arm64: Add kasan_hw_tags_enable() prototype to silence sparse

This function is only called from assembly, no need for a prototype
declaration in a header file. In addition, add #ifdef around the
function since it is only used when CONFIG_KASAN_HW_TAGS.
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: kernel test robot <lkp@intel.com>

arch/arm64/kernel/mte.c

@@ -244,6 +244,11 @@ static void mte_update_gcr_excl(struct task_struct *task)
 		SYS_GCR_EL1);
 }
 
+#ifdef CONFIG_KASAN_HW_TAGS
+/* Only called from assembly, silence sparse */
+void __init kasan_hw_tags_enable(struct alt_instr *alt, __le32 *origptr,
+				 __le32 *updptr, int nr_inst);
+
 void __init kasan_hw_tags_enable(struct alt_instr *alt, __le32 *origptr,
 				 __le32 *updptr, int nr_inst)
 {
@@ -252,6 +257,7 @@ void __init kasan_hw_tags_enable(struct alt_instr *alt, __le32 *origptr,
 	if (kasan_hw_tags_enabled())
 		*updptr = cpu_to_le32(aarch64_insn_gen_nop());
 }
+#endif
 
 void mte_thread_init_user(void)
 {