[gcc] fixes -Wimplicit-fallthrough: common/io.h
[gcc] fixes -Wimplicit-fallthrough: common/io.h

Warning was correct, getFieldType(size, type) with size == 1 or 2 and
type == 'F', would fallthrough and return pcl::PCLPointField::Float32.

drmateo/pcl,drmateo/pcl,drmateo/pcl,drmateo/pcl,drmateo/pcl