stdout-flush-wrapper-debug.sh: Fix bashism.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I5814bcd21414deba37954c60554132c354821b15
diff --git a/stdout-flush-wrapper-debug.sh b/stdout-flush-wrapper-debug.sh
index 00a6231..40567fa 100755
--- a/stdout-flush-wrapper-debug.sh
+++ b/stdout-flush-wrapper-debug.sh
@@ -6,7 +6,7 @@
 # but not completely, so we just give LAVA more time with some sleep after
 # command termination.
 
-if [ "$1" == "--version" ]; then
+if [ "$1" = "--version" ]; then
     echo "$0 1.0"
     exit
 fi