RISC-V: Remove "extern" for namespace [NFC]

RISC-V: Remove "extern" for namespace [NFC]

Just like other targets, aarch64_sve namespace in aarch64-protos.h
arm_mve in arm-protos, nds namesace in nds32-protos.h

They all don't have 'extern' in namespace.
This is a NFC patch to make RISC-V be consistent with other targets.
No functionality change.

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (riscv_run_selftests): Remove 'extern'.
	(init_builtins): Ditto.
	(mangle_builtin_type): Ditto.
	(verify_type_context): Ditto.
	(handle_pragma_vector):  Ditto.
	(builtin_decl): Ditto.
	(expand_builtin): Ditto.
	(const_vec_all_same_in_range_p): Ditto.
	(legitimize_move): Ditto.
	(emit_vlmax_op): Ditto.
	(emit_nonvlmax_op): Ditto.
	(get_vlmul): Ditto.
	(get_ratio): Ditto.
	(get_ta): Ditto.
	(get_ma): Ditto.
	(get_avl_type): Ditto.
	(calculate_ratio): Ditto.
	(enum vlmul_type): Ditto.
	(simm5_p): Ditto.
	(neg_simm5_p): Ditto.
	(has_vi_variant_p): Ditto.