PEL: Add missing gtest dependency

Fixes a compile failure when building the PEL tests in standalone mode
(using subprojects).

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: Ida0abbd824b6573129a3b1262b7b490270ce7787
diff --git a/test/openpower-pels/meson.build b/test/openpower-pels/meson.build
index 923ead6..b6c6a94 100644
--- a/test/openpower-pels/meson.build
+++ b/test/openpower-pels/meson.build
@@ -79,6 +79,7 @@
     dependencies: [
         libpel_deps,
         peltool_deps,
+        gtest_dep,
     ],
 )