Const-correctness in libLTO

Looks like I was sloppy when bridging to C.
Thanks D. Blaikie for noticing!

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 263885
diff --git a/llvm/include/llvm-c/lto.h b/llvm/include/llvm-c/lto.h
index 44f7bea..88b10fd 100644
--- a/llvm/include/llvm-c/lto.h
+++ b/llvm/include/llvm-c/lto.h
@@ -564,7 +564,7 @@
  * \since LTO_API_VERSION=18
  */
 typedef struct {
-  void *Buffer;
+  const char *Buffer;
   size_t Size;
 } LTOObjectBuffer;