netinet/in.h is useless and it does not exist on Win32

Reported by Tobias Doerffel.

src/modbus-data.c

@@ -23,7 +23,6 @@
 #endif
 #include <string.h>
 #include <assert.h>
-#include <netinet/in.h>
 
 /* Sets many bits from a single byte value (all 8 bits of the byte value are
    set) */