backtrace.c: Avoid removed bfd_boolean type.

bfd_boolean has been deprecated for some time and has now been
removed in gdb, so use bool directly instead.

See: 1d95b744c0

Resolves: #2054

main/backtrace.c

@@ -127,7 +127,7 @@ static void process_section(bfd *bfdobj, asection *section, void *obj)
 	bfd_vma offset;
 	bfd_vma vma;
 	bfd_size_type size;
-	bfd_boolean line_found = 0;
+	bool line_found = 0;
 	const char *fn;
 	int inlined = 0;