bench: explicitly make all current benchmarks "high" priority

no-functional changes. Only have set the priority level explicitly
on every BENCHMARK macro call.

src/bench/poly1305.cpp

@@ -36,6 +36,6 @@ static void POLY1305_1MB(benchmark::Bench& bench)
     POLY1305(bench, BUFFER_SIZE_LARGE);
 }
 
-BENCHMARK(POLY1305_64BYTES);
-BENCHMARK(POLY1305_256BYTES);
-BENCHMARK(POLY1305_1MB);
+BENCHMARK(POLY1305_64BYTES, benchmark::PriorityLevel::HIGH);
+BENCHMARK(POLY1305_256BYTES, benchmark::PriorityLevel::HIGH);
+BENCHMARK(POLY1305_1MB, benchmark::PriorityLevel::HIGH);