fix box2d compile error in ndk

external/Box2D/Collision/Shapes/b2ChainShape.cpp

@@ -20,6 +20,7 @@
 #include <Box2D/Collision/Shapes/b2EdgeShape.h>
 #include <new>
 #include <memory.h>
+#include <string.h>
 
 b2ChainShape::~b2ChainShape()
 {

external/Box2D/Collision/b2DynamicTree.cpp

@@ -18,6 +18,7 @@
 
 #include <Box2D/Collision/b2DynamicTree.h>
 #include <memory.h>
+#include <string.h>
 
 b2DynamicTree::b2DynamicTree()
 {

external/Box2D/Common/b2BlockAllocator.cpp

@@ -20,6 +20,7 @@
 #include <limits.h>
 #include <memory.h>
 #include <stddef.h>
+#include <string.h>
 
 int32 b2BlockAllocator::s_blockSizes[b2_blockSizes] = 
 {

external/Box2D/Common/b2GrowableStack.h

@@ -20,6 +20,7 @@
 #define B2_GROWABLE_STACK_H
 #include <Box2D/Common/b2Settings.h>
 #include <memory.h>
+#include <string.h>
 
 /// This is a growable LIFO stack with an initial capacity of N.
 /// If the stack size exceeds the initial capacity, the heap is used