Add trailing slash to glmark2 data dep

glmark2 is being included as part of angle_perftests, which is built as
an isolate.

Isolates do not work correctly unless there is a trailing slash for
folders in the data section.

Bug: angleproject:3311
Change-Id: I55e5a655ae15683d5cb070589970ba5d9eba4da0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1538676
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>

third_party/glmark2/BUILD.gn

@@ -341,7 +341,7 @@ template("glmark2_exe") {
       "src/src/main.cpp",
     ]
     data = [
-      "src/data",
+      "src/data/",
     ]
 
     defines += [ "GLMARK_VERSION=\"${glmark2_version}\"" ]