mobile/bind/java: fix compiler warnings

Add a missing #include to declare the exported Go function
setContext, and replace old GNU-style struct initializers.

Change-Id: Id1660559236c39505a47368a700c8e0ad834cf6c
Reviewed-on: https://go-review.googlesource.com/24491
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/java/context_android.c

@@ -4,6 +4,7 @@
 
 #include <jni.h>
 #include "seq.h"
+#include "_cgo_export.h"
 
 JNIEXPORT void JNICALL
 Java_go_Seq_setContext(JNIEnv* env, jclass clazz, jobject ctx) {