Put all the thresholding code in the same place

This will make it easier to write tests validating the fast version.

Change-Id: I27cc372cc848c495d10a138b3c149d550dc0dad9
diff --git a/aos/vision/blob/BUILD b/aos/vision/blob/BUILD
index 69a2cc5..d22aa4d 100644
--- a/aos/vision/blob/BUILD
+++ b/aos/vision/blob/BUILD
@@ -37,7 +37,12 @@
 
 cc_library(
     name = "threshold",
-    hdrs = ["threshold.h"],
+    srcs = [
+        "threshold.cc",
+    ],
+    hdrs = [
+        "threshold.h",
+    ],
     deps = [
         ":range_image",
         "//aos/vision/image:image_types",