trace: declare blk_add_trace_rq non-static on OL6

trace: declare blk_add_trace_rq non-static on OL6

In an external module, there is a reference to this function done
via explicit symbol lookup. The newer compiler used for retpoline
support on OL6 changes the symbol's asm name, which causes this lookup
to fail.

While it doesn't guarantee kABI compatibility, remove the "static"
qualifier for this function, on OL6 only, to allow continuing usage of
this code in the short term.

Orabug: 27578618
Signed-off-by: Todd Vierling <todd.vierling@oracle.com>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>