[gdb/testsuite] Use set in lmap in gdb.dwarf2/dw2-abs-hi-pc.exp

In gdb.dwarf2/dw2-abs-hi-pc.exp we do:
...
set sources [lmap i $sources { expr { "$srcdir/$subdir/$i" } }]
...

The use of expr is not idiomatic.  Fix this by using set instead:
...
set sources [lmap i $sources { set tmp $srcdir/$subdir/$i }]
...

Reported-By: Tom Tromey <tom@tromey.com>
Reviewed-By: Andreas Schwab <schwab@suse.de>