PR bootstrap/106472: Add libgo depends on libbacktrace to Makefile.def

This patch fixes PR bootstrap/106472 by adding a missing dependency
to Makefile.def to allow make bootstrap when configured using
"--enable-languages=go" (and not using make with multiple threads).

2022-07-31  Roger Sayle  <roger@nextmovesoftware.com>

ChangeLog
	PR bootstrap/106472
	* Makefile.def (dependencies): Make configure-target-libgo depend
	upon all-target-libbacktrace.