libavutil: Fix building libavutil/tests/cpu for aarch64 without SVE

This adds a condition that was missed in
b1ee2af843, fixing compilation on
toolchains that don't support SVE.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/tests/cpu.c

@@ -165,7 +165,7 @@ int main(int argc, char **argv)
     print_cpu_flags(cpu_flags_raw, "raw");
     print_cpu_flags(cpu_flags_eff, "effective");
     printf("threads = %s (cpu_count = %d)\n", threads, cpu_count);
-#if ARCH_AARCH64
+#if ARCH_AARCH64 && HAVE_SVE
     if (cpu_flags_raw & AV_CPU_FLAG_SVE)
         printf("sve_vector_length = %d\n", 8 * ff_aarch64_sve_length());
 #endif