Fix building when byteswap.h is not defined
Fix building when byteswap.h is not defined

Linking failed in Visual Studio due to unresolved bswap_16().

stephane/libmodbus,wablair/libmodbus,marinbek/libmodbus,stephane/libmodbus,giangdo/modbusMSim,charlie-x/libmodbus,hmoraes/libmodbus,mhei/libmodbus,wablair/libmodbus,dhruvvyas90/libmodbus,stephane/libmodbus,charlie-x/libmodbus,mhei/libmodbus,dhruvvyas90/libmodbus,OliverDDS/libmodbus,mhei/libmodbus,marinbek/libmodbus,marinbek/libmodbus,OliverDDS/libmodbus,OliverDDS/libmodbus,wablair/libmodbus,dhruvvyas90/libmodbus,charlie-x/libmodbus,hmoraes/libmodbus,hmoraes/libmodbus