Fix compilation on Linux

BUG=angleproject:892

Change-Id: I135e356e1ec78c95e69a260bd0576e5285e759b5
Reviewed-on: https://chromium-review.googlesource.com/283225
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/libANGLE/Program.cpp b/src/libANGLE/Program.cpp
index b472201..05b9691 100644
--- a/src/libANGLE/Program.cpp
+++ b/src/libANGLE/Program.cpp
@@ -1611,6 +1611,7 @@
 
         // All transform feedback varyings are expected to exist since packVaryings checks for them.
         ASSERT(found);
+        UNUSED_ASSERTION_VARIABLE(found);
     }
 
     if (transformFeedbackBufferMode == GL_INTERLEAVED_ATTRIBS &&