MIPS: init: move externs to header file

This commit fixes the following checkpatch warnings:

    WARNING: externs should be avoided in .c files

    This is a warning for placing declarations in a ".c" file.
    This fix removes the declaration in ".c" and adds it to
    the common header file.
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Huacai Chen <chenhuacai@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/traps.h

@@ -24,6 +24,7 @@ extern void (*board_ebase_setup)(void);
 extern void (*board_cache_error_setup)(void);
 
 extern int register_nmi_notifier(struct notifier_block *nb);
+extern char except_vec_nmi[];
 
 #define nmi_notifier(fn, pri)						\
 ({									\

arch/mips/loongson2ef/common/init.c

@@ -19,7 +19,6 @@ unsigned long __maybe_unused _loongson_addrwincfg_base;
 static void __init mips_nmi_setup(void)
 {
 	void *base;
-	extern char except_vec_nmi[];
 
 	base = (void *)(CAC_BASE + 0x380);
 	memcpy(base, except_vec_nmi, 0x80);

arch/mips/loongson64/init.c

@@ -25,7 +25,6 @@ u32 node_id_offset;
 static void __init mips_nmi_setup(void)
 {
 	void *base;
-	extern char except_vec_nmi[];
 
 	base = (void *)(CAC_BASE + 0x380);
 	memcpy(base, except_vec_nmi, 0x80);

arch/mips/mti-malta/malta-init.c

@@ -90,7 +90,6 @@ static void __init console_config(void)
 static void __init mips_nmi_setup(void)
 {
 	void *base;
-	extern char except_vec_nmi[];
 
 	base = cpu_has_veic ?
 		(void *)(CAC_BASE + 0xa80) :

arch/mips/pistachio/init.c

@@ -83,7 +83,6 @@ phys_addr_t mips_cdmm_phys_base(void)
 static void __init mips_nmi_setup(void)
 {
 	void *base;
-	extern char except_vec_nmi[];
 
 	base = cpu_has_veic ?
 		(void *)(CAC_BASE + 0xa80) :