Fix starboard_platform_tests install target dependency (#3501)

b/345848337

starboard_platform_tests_install was being excluded from the build graph
as it didn't have any target depending on it.
This change adds a dependency on "starboard_platform_tests_install"

Test-On-Device: true

starboard/BUILD.gn

@@ -39,7 +39,10 @@ group("gn_all") {
   }
 
   if (platform_tests_path == "") {
-    deps += [ ":starboard_platform_tests($starboard_toolchain)" ]
+    deps += [
+      ":starboard_platform_tests($starboard_toolchain)",
+      ":starboard_platform_tests_install($starboard_toolchain)",
+    ]
   } else {
     deps += [ platform_tests_path ]
   }