qemu: fix out of tree cross compile

The symlink to "asm" platform linux headers is made in the build tree by
the configure script but gcc is not told to look for them there.

The patch fixes this.
Signed-off-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

configure

@@ -546,7 +546,7 @@ Haiku)
   if [ "$cpu" = "i386" -o "$cpu" = "x86_64" ] ; then
     audio_possible_drivers="$audio_possible_drivers fmod"
   fi
-  QEMU_INCLUDES="-I\$(SRC_PATH)/linux-headers $QEMU_INCLUDES"
+  QEMU_INCLUDES="-I\$(SRC_PATH)/linux-headers -I$(pwd)/linux-headers $QEMU_INCLUDES"
 ;;
 esac