Add missing static keywords.

[jpidancet]

Fix some warnings output by GCC when compiling amalgamated source:

Zydis.c:45353:12: warning: no previous declaration for ‘ZydisDecodeOperands’ [-Wmissing-declarations] 45353 | ZyanStatus ZydisDecodeOperands(const ZydisDecoder decoder, const ZydisDecoderContext context, | ^~~~~~~ Zydis.c:52494:6: warning: no previous declaration for ‘ZydisBuildRegisterOperand’ [-Wmissing-declarations] 52494 | void ZydisBuildRegisterOperand(const ZydisEncoderOperand user_op, | ^~~~~~~~~ Zydis.c:53639:6: warning: no previous declaration for ‘ZydisFormatterBufferInit’ [-Wmissing-declarations] 53639 | void ZydisFormatterBufferInit(ZydisFormatterBuffer buffer, char user_buffer, | ^~~~~~~~ Zydis.c:53661:6: warning: no previous declaration for ‘ZydisFormatterBufferInitTokenized’ [-Wmissing-declarations] 53661 | void ZydisFormatterBufferInitTokenized(ZydisFormatterBuffer buffer, | ^~~~~~~~~ Zydis.c:54354:12: warning: no previous declaration for ‘ZydisStringAppendDecU64’ [-Wmissing-declarations] 54354 | ZyanStatus ZydisStringAppendDecU64(ZyanString string, ZyanU64 value, ZyanU8 padding_length) | ^~~~~~~ Zydis.c:54475:12: warning: no previous declaration for ‘ZydisStringAppendHexU64’ [-Wmissing-declarations] 54475 | ZyanStatus ZydisStringAppendHexU64(ZyanString string, ZyanU64 value, ZyanU8 padding_length, | ^~~~~~~