kbuild,gcov: simplify kernel/gcov/Makefile

Kbuild descends into kernel/gcov/ directory only when
CONFIG_GCOV_KERNEL is enabled. (See kernel/Makefile)

CONFIG_GCOV_KERNEL check can be omitted in kernel/gcov/Makefile.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>