cgo: add typedef for uintptr in generated headers

Fixes #1046.

R=rsc, iant
CC=golang-dev
https://golang.org/cl/2102043

src/cmd/cgo/out.go

@@ -631,6 +631,7 @@ typedef unsigned char uchar;
 typedef unsigned short ushort;
 typedef long long int64;
 typedef unsigned long long uint64;
+typedef __SIZE_TYPE__ uintptr;
 
 typedef struct { char *p; int n; } GoString;
 typedef void *GoMap;