Config.in.legacy: add missing select

Commit 5c89b00a2052 (package/gdb: remove support for 9.2) forgot to
select the legacy symbol when gdb 9.2 was previously enabled.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

Config.in.legacy

@@ -154,6 +154,7 @@ config BR2_PACKAGE_GPSD_OLDSTYLE
 
 config BR2_GDB_VERSION_9_2
 	bool "gdb 9.2 removed"
+	select BR2_LEGACY
 	help
 	  Support for GDB 9.2 has been removed. A new version has
 	  automatically been selected.