Actually disable background compilation for benchmarks

The flag should be a `-` since [the
intention](mozilla@efe57ab#diff-77ab8968a72299a5fe665cc05d74e3f4c8d3059b243f8fc52d94462a898909fbR35)
is to disable background compilation, but it was accidentally a `+`.

Author: andreabergia

benchmarks/build.gradle

@@ -32,7 +32,7 @@ jmh {
             }
         }
     }
-    jvmArgs = ['-XX:+BackgroundCompilation']
+    jvmArgs = ['-XX:-BackgroundCompilation']
     benchmarkMode = ['avgt']
     fork = 1
     iterations = 5