libclang: Mark clang_Cursor_isNull with CINDEX_LINKAGE to make it available on windows.

Patch by Timothy J Fontaine!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@149533 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/clang-c/Index.h b/include/clang-c/Index.h
index 588d93a..a5082d1 100644
--- a/include/clang-c/Index.h
+++ b/include/clang-c/Index.h
@@ -1984,7 +1984,7 @@
 /**
  * \brief Returns non-zero if \arg cursor is null.
  */
-int clang_Cursor_isNull(CXCursor);
+CINDEX_LINKAGE int clang_Cursor_isNull(CXCursor);
 
 /**
  * \brief Compute a hash value for the given cursor.