gst-inspect: fix printing the first field of a GstStructure

gst-inspect: fix printing the first field of a GstStructure

When printing a GstStructure property (e.g. the "stats" property in
rtpsession) the first field is printed on the same line of the type
description, and this is both inconsistent compared to  how Enum values
are printed and confusing as the reader might miss the first field.

To fix this, add a newline before printing GstStructure fields in
properties.

NOTE: this does not change the existing inconsistent behavior of an
extra newline *after* a GstStructure property, but the latter is not as
annoying and it would take more effort to fix because GstStructure
fields are printed in CAPS descriptions too.