toolchain/toolchain-external/toolchain-external-custom: add gcc 15 version selection

This patch allows to use an external toolchain based on gcc 15.

Signed-off-by: Edgar Bonet <bonet@grenoble.cnrs.fr>
[Julien: change spaces to tabs to fix check-package errors]
Signed-off-by: Julien Olivain <ju.o@free.fr>

toolchain/toolchain-external/toolchain-external-custom/Config.in.options

@@ -46,6 +46,10 @@ choice
 	  Set to the gcc version that is used by your external
 	  toolchain.
 
+config BR2_TOOLCHAIN_EXTERNAL_GCC_15
+	bool "15.x"
+	select BR2_TOOLCHAIN_GCC_AT_LEAST_15
+
 config BR2_TOOLCHAIN_EXTERNAL_GCC_14
 	bool "14.x"
 	select BR2_TOOLCHAIN_GCC_AT_LEAST_14