Mark internal implementation functions as hidden

Adding hidden attribute results in compilation failure if the symbol is
not found when linking the .so. It should also lead to better
performance, as it side-steps GOT.

Change-Id: I7b6f39e663ca2b3e432669a3e2b36d7395f555b6
Signed-off-by: Robert Varga <nite@hq.sk>
diff --git a/vpp-japi/japi/vppjni_env.h b/vpp-japi/japi/vppjni_env.h
index fe9767b..44029c2 100644
--- a/vpp-japi/japi/vppjni_env.h
+++ b/vpp-japi/japi/vppjni_env.h
@@ -110,9 +110,9 @@
 #define BIND_JAPI_STRING_FIELD(cls, field) \
     BIND_JAPI_OBJ_FIELD(cls, field, "Ljava/lang/String;")
 
-jobject vppjni_new_object(JNIEnv *env, const vppjni_class_t *ptr, va_list ap);
-void vppjni_register_class(vppjni_class_t *ptr);
-void vppjni_register_field(vppjni_field_t *ptr);
-int vppjni_init(JNIEnv *env);
-void vppjni_uninit(JNIEnv *env);
+jobject vppjni_new_object(JNIEnv *env, const vppjni_class_t *ptr, va_list ap) __attribute__ ((visibility ("hidden")));
+void vppjni_register_class(vppjni_class_t *ptr) __attribute__ ((visibility ("hidden")));
+void vppjni_register_field(vppjni_field_t *ptr) __attribute__ ((visibility ("hidden")));
+int vppjni_init(JNIEnv *env) __attribute__ ((visibility ("hidden")));
+void vppjni_uninit(JNIEnv *env) __attribute__ ((visibility ("hidden")));