Cast pointers through uintptr_t under JNI

Fixes warnings of the form "warning: cast to pointer from integer of
different size" when building on 32 bit platforms. This is the same
approach used for pointer conversions in the openjdk sources.

src/java/org_bitcoin_Secp256k1Context.c

@@ -1,4 +1,5 @@
 #include <stdlib.h>
+#include <stdint.h>
 #include "org_bitcoin_Secp256k1Context.h"
 #include "include/secp256k1.h"
 
@@ -9,6 +10,6 @@ SECP256K1_API jlong JNICALL Java_org_bitcoin_Secp256k1Context_secp256k1_1init_1c
 
   (void)classObject;(void)env;
 
-  return (jlong)ctx;
+  return (uintptr_t)ctx;
 }