toolchain: external 3.17 headers typo fix

Commit 2d312b7b61c98716535513a2743a85d4d7e8a084 had a typo
"." instead of "_" in BR2_TOOLCHAIN_HEADERS_AT_LEAST_3.17.
This made selecting 3.17 as custom external headers version
impossible.

Signed-off-by: Karoly Kasza <kaszak@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

toolchain/toolchain-external/Config.in

@@ -994,7 +994,7 @@ choice
 
 config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_17
 	bool "3.17.x"
-	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3.17
+	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_17
 
 config BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_16
 	bool "3.16.x"