arm64: vdso: Fix "no previous prototype" warning

arm64: vdso: Fix "no previous prototype" warning

If compiling the arm64 kernel with W=1 the following warning is produced:

| arch/arm64/kernel/vdso/vgettimeofday.c:9:5: error: no previous prototype for ‘__kernel_clock_gettime’ [-Werror=missing-prototypes]
|     9 | int __kernel_clock_gettime(clockid_t clock,
|       |     ^~~~~~~~~~~~~~~~~~~~~~
| arch/arm64/kernel/vdso/vgettimeofday.c:15:5: error: no previous prototype for ‘__kernel_gettimeofday’ [-Werror=missing-prototypes]
|    15 | int __kernel_gettimeofday(struct __kernel_old_timeval *tv,
|       |     ^~~~~~~~~~~~~~~~~~~~~
| arch/arm64/kernel/vdso/vgettimeofday.c:21:5: error: no previous prototype for ‘__kernel_clock_getres’ [-Werror=missing-prototypes]
|    21 | int __kernel_clock_getres(clockid_t clock_id,
|       |     ^~~~~~~~~~~~~~~~~~~~~

This patch removes "-Wmissing-prototypes" and "-Wmissing-declarations" compilers
flags from the compilation of vgettimeofday.c to make possible to build the
kernel with CONFIG_WERROR enabled.

Cc: Will Deacon <will@kernel.org>
Reported-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Tested-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/20220121121234.47273-1-vincenzo.frascino@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>