tools/rtla: Fix installation from out-of-tree build

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>

tools/tracing/rtla/Makefile.rtla

@@ -38,7 +38,7 @@ BINDIR		:= /usr/bin
 .PHONY: install
 install: doc_install
 	@$(MKDIR) -p $(DESTDIR)$(BINDIR)
-	$(call QUIET_INSTALL,rtla)$(INSTALL) rtla -m 755 $(DESTDIR)$(BINDIR)
+	$(call QUIET_INSTALL,rtla)$(INSTALL) $(RTLA) -m 755 $(DESTDIR)$(BINDIR)
 	@$(STRIP) $(DESTDIR)$(BINDIR)/rtla
 	@test ! -f $(DESTDIR)$(BINDIR)/osnoise || $(RM) $(DESTDIR)$(BINDIR)/osnoise
 	@$(LN) rtla $(DESTDIR)$(BINDIR)/osnoise