phosphor-software-manager: Fix tests meson param

Need to append the meson option to disable tests so that it doesn't
overwrite previous options.

Tested: Built witherspoon with -e option and verified EXTRA_OEMESON
showed all the meson options for ubi.

Change-Id: I1275a790feefdc3aa5d85aa9b36bf5d7d3a4d44e
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/meta-phosphor/recipes-phosphor/flash/phosphor-software-manager_git.bb b/meta-phosphor/recipes-phosphor/flash/phosphor-software-manager_git.bb
index 19e5f31..4b0bdc0 100644
--- a/meta-phosphor/recipes-phosphor/flash/phosphor-software-manager_git.bb
+++ b/meta-phosphor/recipes-phosphor/flash/phosphor-software-manager_git.bb
@@ -84,7 +84,7 @@
 
 S = "${WORKDIR}/git"
 
-EXTRA_OEMESON = "-Dtests=disabled"
+EXTRA_OEMESON += "-Dtests=disabled"
 
 do_install_append() {
     install -d ${D}/usr/local