Makefile.kcov: apply needed compiler option unconditionally in CFLAGS_KCOV

Commit 852faf80 ("gcc-plugins: remove SANCOV gcc plugin") removes the
config CC_HAS_SANCOV_TRACE_PC, as all supported compilers include the
compiler option '-fsanitize-coverage=trace-pc' by now.

The commit however misses the important use of this config option in
Makefile.kcov to add '-fsanitize-coverage=trace-pc' to CFLAGS_KCOV.
Include the compiler option '-fsanitize-coverage=trace-pc' unconditionally
to CFLAGS_KCOV, as all compilers provide that option now.

Fixes: 852faf80 ("gcc-plugins: remove SANCOV gcc plugin")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>