fix alignment problem (build fails with -Werror and -Wcast-align on ARMv...

...5 platform)

On older ARM processors you have to keep in mind CPU can access memory 32bit aligned only. Cross compiling libmodbus with ptxdist and the additional flag -Wcast-align leads to a failing build because libmodbus uses -Werror. This fixes the alignment issue. Tree cross-compiles with these changes, tested however on i386 with Debian Wheezy only.