Suppress harmless -Wmissing-fields-initialize in wxSecretStore

This warning is difficult to avoid as we don't want to initialize the
unused/reserved fields of SecretSchema struct, yet the compiler warns
about it (when using -Wextra).

src/unix/secretstore.cpp

@@ -235,6 +235,11 @@ private:
     // helper function to make changing the code later simpler.
     static SecretSchema* GetSchema()
     {
+        // SecretSchema struct has some "reserved" fields in it which we don't
+        // want to initialize, but this results in this warning if it's
+        // enabled, so just suppress it here.
+        wxGCC_WARNING_SUPPRESS(missing-field-initializers)
+
         static SecretSchema s_schema =
             {
                 "org.freedesktop.Secret.Generic",
@@ -246,6 +251,8 @@ private:
                 }
             };
 
+        wxGCC_WARNING_RESTORE(missing-field-initializers)
+
         return &s_schema;
     }