selftests: proc: mark vsyscall strings maybe-unused

selftests: proc: mark vsyscall strings maybe-unused

The str_vsyscall_* constants in proc-pid-vm.c triggers
-Wunused-const-variable warnings with gcc-13.32 and clang 18.1.

Define and apply __maybe_unused locally to suppress the warnings.  No
functional change

Fixes compiler warning:
warning: `str_vsyscall_*' defined but not used[-Wunused-const-variable]

Link: https://lkml.kernel.org/r/20250820175610.83014-1-reddybalavignesh9979@gmail.com
Signed-off-by: Bala-Vignesh-Reddy <reddybalavignesh9979@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>