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: NVasily Gorbik <gor@linux.vnet.ibm.com>
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>

arch/s390/Kconfig

@@ -148,6 +148,7 @@ config S390
 	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