Fix size test flag name (#497)

Summary:
Pull Request resolved: #497

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Reviewed By: larryliu0820

Differential Revision: D49657774

fbshipit-source-id: 2752aa7eaecf3dcb9065aeff43e34368a5076e1b

Author: lucylq

test/build_size_test.sh

@@ -16,7 +16,7 @@ test_cmake_size_test() {
         && mkdir cmake-out \
         && cd cmake-out \
         && retry cmake -DBUCK2="$BUCK2" \
-            -DBUILD_SIZE_TEST=ON \
+            -DEXECUTORCH_BUILD_SIZE_TEST=ON \
             -DCMAKE_BUILD_TYPE=Release \
             -DPYTHON_EXECUTABLE="$PYTHON_EXECUTABLE" ..)