Ensure unittests are built when building all.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/849113002
diff --git a/build/all.gyp b/build/all.gyp
index c97871d..7e2ea61 100644
--- a/build/all.gyp
+++ b/build/all.gyp
@@ -8,6 +8,7 @@
       'target_name': 'All',
       'type': 'none',
       'dependencies': [
+        '../pdfium.gyp:*',
         '../samples/samples.gyp:*',
       ],
     }