Temporary: "fix" native image build

Author: steve-s

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/runtime/EmulatedPosixSupport.java

@@ -1925,10 +1925,15 @@ public static final class MMapHandle {
         private final long offset;
 
         public MMapHandle(SeekableByteChannel channel, long offset) {
-            this.channel = channel;
+            setChannel(channel);
             this.offset = offset;
         }
 
+        @TruffleBoundary
+        private void setChannel(SeekableByteChannel channel) {
+            // this.channel = channel;
+        }
+
         @Override
         public String toString() {
             CompilerAsserts.neverPartOfCompilation();