Config.in: ensure BR2_SSP_STRONG can only be selected if supported

This commit ensures that BR2_SSP_STRONG cannot be chosen if the
toolchain doesn't support strong SSP.

Fixes:

  http://autobuild.buildroot.net/results/cba93a681d10692c4e4c5584e4c962bd18a608d4/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

Config.in

@@ -758,6 +758,7 @@ config BR2_SSP_REGULAR
 
 config BR2_SSP_STRONG
 	bool "-fstack-protector-strong"
+	depends on BR2_TOOLCHAIN_HAS_SSP_STRONG
 	help
 	  Like -fstack-protector but includes additional functions to be
 	  protected - those that have local array definitions, or have