rtla now supports out-of-tree builds, but installation fails as it
still tries to install the rtla binary from the source tree.  Use the
existing macro $(RTLA) to refer to the binary.

Link: https://lore.kernel.org/ZudubuoU_JHjPZ7w@decadent.org.uk
Fixes: 01474dc7 ("tools/rtla: Use tools/build makefiles to build rtla")
Reviewed-by: Tomas Glozar <tglozar@redhat.com>
Tested-by: Tomas Glozar <tglozar@redhat.com>
Signed-off-by: Ben Hutchings <benh@debian.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>