s390/kbuild: get rid of a warning when compiling with KCOV

This change fixes the following warning:
warning: (KCOV) selects GCC_PLUGINS which has unmet direct dependencies
(HAVE_GCC_PLUGINS && !COMPILE_TEST)

Signed-off-by: Vasily Gorbik <gor@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 863a62a..f5beccb 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -148,6 +148,7 @@
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER
 	select HAVE_FUTEX_CMPXCHG if FUTEX
+	select HAVE_GCC_PLUGINS
 	select HAVE_KERNEL_BZIP2
 	select HAVE_KERNEL_GZIP
 	select HAVE_KERNEL_LZ4