gdb: move target_desc_info to inferior.h

gdb: move target_desc_info to inferior.h

In preparation for the following patch, where struct inferior needs to
"see" struct target_desc_info, move target_desc_info to the header file.

I initially moved the structure to target-descriptions.h, and later made
inferior.h include target-descriptions.h.  This worked, but it then
occured to me that target_desc_info is really an inferior property that
involves a target description, so I think it makes sense to have it in
inferior.h.

Change-Id: I3e81d04faafcad431e294357389f3d4c601ee83d