plugins: add strstr() to plugin API

Multiple plugins carry their own strstr() implementation because
the in-core version wasn't exported to the plugin API.

Change-Id: Ib57a9b63754c89fb3447ec1d3958963d23145105

apps/plugin.h

@@ -1019,6 +1019,7 @@ struct plugin_api {
                                 const struct touchevent *ev);
     bool (*gesture_vel_get)(struct gesture_vel *gv, int *xvel, int *yvel);
 #endif
+    char* (*strstr)(const char *s1, const char *s2);
 };
 
 /* plugin header */