Fix typo in ASAN CI build (#3623)

diff --git a/kokoro/scripts/linux/build.sh b/kokoro/scripts/linux/build.sh
index 8fb7bdd..3a87646 100644
--- a/kokoro/scripts/linux/build.sh
+++ b/kokoro/scripts/linux/build.sh
@@ -46,7 +46,7 @@
 ADDITIONAL_CMAKE_FLAGS=""
 if [ $CONFIG = "ASAN" ]
 then
-  ADDITIONAL_CMAKE_FLAGS="SPIRV_USE_SANITIZER=address,bounds,null"
+  ADDITIONAL_CMAKE_FLAGS="-DSPIRV_USE_SANITIZER=address,bounds,null"
   [ $COMPILER = "clang" ] || { echo "$CONFIG requires clang"; exit 1; }
 elif [ $CONFIG = "COVERAGE" ]
 then