gcov: shut up missing prototype warnings for internal stubs

gcov uses global functions that are called from generated code, but these
have no prototype in a header, which causes a W=1 build warning:

kernel/gcov/gcc_base.c:12:6: error: no previous prototype for '__gcov_init' [-Werror=missing-prototypes]
kernel/gcov/gcc_base.c:40:6: error: no previous prototype for '__gcov_flush' [-Werror=missing-prototypes]
kernel/gcov/gcc_base.c:46:6: error: no previous prototype for '__gcov_merge_add' [-Werror=missing-prototypes]
kernel/gcov/gcc_base.c:52:6: error: no previous prototype for '__gcov_merge_single' [-Werror=missing-prototypes]

Just turn off these warnings unconditionally for the two files that
contain them.

Link: https://lore.kernel.org/all/0820010f-e9dc-779d-7924-49c7df446bce@linux.ibm.com/
Link: https://lkml.kernel.org/r/20230725123042.2269077-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Acked-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>