Update modbus-data.c

Making all in src
  CC       modbus.lo
  CC       modbus-data.lo
modbus-data.c:51:5: attention : #warning "Fallback on C functions for bswap_16" [-Wcpp]
 #   warning "Fallback on C functions for bswap_16"
     ^
modbus-data.c:52:24: erreur: redefinition of ‘bswap_16’
 static inline uint16_t bswap_16(uint16_t x)

src/modbus-data.c

@@ -47,6 +47,7 @@
 # define bswap_16 _byteswap_ushort
 #endif
 
+#if !defined(__CYGWIN__)
 #if !defined(bswap_16)
 #   warning "Fallback on C functions for bswap_16"
 static inline uint16_t bswap_16(uint16_t x)
@@ -54,6 +55,7 @@ static inline uint16_t bswap_16(uint16_t x)
     return (x >> 8) | (x << 8);
 }
 #endif
+#endif
 
 #if !defined(bswap_32)
 #   warning "Fallback on C functions for bswap_32"