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>