riscv: Fix "no previous prototype" compile warning in kgdb.c file

Some functions are only used in the kgdb.c file. Add static properities
to these functions to avoid "no previous prototype" compile warnings
Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/kgdb.c

@@ -44,18 +44,18 @@ DECLARE_INSN(c_beqz, MATCH_C_BEQZ, MASK_C_BEQZ)
 DECLARE_INSN(c_bnez, MATCH_C_BNEZ, MASK_C_BNEZ)
 DECLARE_INSN(sret, MATCH_SRET, MASK_SRET)
 
-int decode_register_index(unsigned long opcode, int offset)
+static int decode_register_index(unsigned long opcode, int offset)
 {
 	return (opcode >> offset) & 0x1F;
 }
 
-int decode_register_index_short(unsigned long opcode, int offset)
+static int decode_register_index_short(unsigned long opcode, int offset)
 {
 	return ((opcode >> offset) & 0x7) + 8;
 }
 
 /* Calculate the new address for after a step */
-int get_step_address(struct pt_regs *regs, unsigned long *next_addr)
+static int get_step_address(struct pt_regs *regs, unsigned long *next_addr)
 {
 	unsigned long pc = regs->epc;
 	unsigned long *regs_ptr = (unsigned long *)regs;
@@ -136,7 +136,7 @@ int get_step_address(struct pt_regs *regs, unsigned long *next_addr)
 	return 0;
 }
 
-int do_single_step(struct pt_regs *regs)
+static int do_single_step(struct pt_regs *regs)
 {
 	/* Determine where the target instruction will send us to */
 	unsigned long addr = 0;
@@ -320,7 +320,7 @@ int kgdb_arch_handle_exception(int vector, int signo, int err_code,
 	return err;
 }
 
-int kgdb_riscv_kgdbbreak(unsigned long addr)
+static int kgdb_riscv_kgdbbreak(unsigned long addr)
 {
 	if (stepped_address == addr)
 		return KGDB_SW_SINGLE_STEP;