trace: fix out-of-tree builds

Reported-by: NLluis Vilanova <vilanova@ac.upc.edu>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -3617,7 +3617,7 @@ DIRS="$DIRS pc-bios/spapr-rtas"
 DIRS="$DIRS roms/seabios roms/vgabios"
 DIRS="$DIRS fsdev ui"
 DIRS="$DIRS qapi"
-DIRS="$DIRS qga"
+DIRS="$DIRS qga trace"
 FILES="Makefile tests/Makefile"
 FILES="$FILES tests/cris/Makefile tests/cris/.gdbinit"
 FILES="$FILES pc-bios/optionrom/Makefile pc-bios/keymaps"