add the jni initializer to the build for sampling profiler

Summary: The OnLoad.cpp file is needed since it actually registers the jni functions, it wasn't included in the build so it wasn't working correctly.

Reviewed By: jbower-fb

Differential Revision: D16826230

fbshipit-source-id: 0243e456c4015879d17650737a6a27a58a3d0d9a

ReactAndroid/src/main/java/com/facebook/hermes/instrumentation/BUCK

@@ -22,7 +22,10 @@ rn_android_library(
 
 rn_xplat_cxx_library(
     name = "jni_hermes_samplingprofiler",
-    srcs = ["HermesSamplingProfiler.cpp"],
+    srcs = [
+        "HermesSamplingProfiler.cpp",
+        "OnLoad.cpp",
+    ],
     headers = ["HermesSamplingProfiler.h"],
     header_namespace = "",
     #allow_jni_merging = True,