[libclang] Add clang_getFileContents to libclang.exports

This is the follow up patch for rL319881 which introduced
the function but did not put it into .exports file.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@321695 91177308-0d34-0410-b5e6-96231b3b80d8

tools/libclang/libclang.exports

@@ -220,6 +220,7 @@ clang_getExceptionSpecificationType
 clang_getFieldDeclBitWidth
 clang_getExpansionLocation
 clang_getFile
+clang_getFileContents
 clang_getFileLocation
 clang_getFileName
 clang_getFileTime