Exclude viewer from NoGpu bots

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3623

Change-Id: Ie1146a7092e08b83e5a332a26d771652f977b52a
Reviewed-on: https://skia-review.googlesource.com/3623
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 4342d0f..25985e1 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1174,7 +1174,7 @@
     testonly = true
   }
 
-  if (is_linux || is_win) {
+  if (skia_enable_gpu && (is_linux || is_win)) {
     executable("viewer") {
       sources = [
         "tools/viewer/GMSlide.cpp",