Add prototype for rs_mdfour_block, and make it static.
Add prototype for rs_mdfour_block, and make it static.

On i386 rs_mdfour_block can be an inline-able static function rather
than a macro, just for consistency.

yrgoldteeth/librsync,sourcefrog/librsync,librsync/librsync,sourcefrog/librsync,binondord/librsync,yrgoldteeth/librsync,agrandville/librsync,librsync/librsync,dbaarda/librsync,potatogim/librsync,Salamek/librsync,pauloppenheim-liveloop/librsync,yrgoldteeth/librsync,binondord/librsync,librsync/librsync,sourcefrog/librsync,Salamek/librsync,Salamek/librsync,dbaarda/librsync,binondord/librsync,VictorDenisov/librsync,pauloppenheim-liveloop/librsync-winbuild,potatogim/librsync,pauloppenheim-liveloop/librsync-winbuild,dbaarda/librsync,VictorDenisov/librsync,pauloppenheim-liveloop/librsync,sourcefrog/librsync,librsync/librsync,agrandville/librsync,yrgoldteeth/librsync,dbaarda/librsync,VictorDenisov/librsync,potatogim/librsync,pauloppenheim-liveloop/librsync-winbuild,pauloppenheim-liveloop/librsync,agrandville/librsync,VictorDenisov/librsync