[release] Add third-party tarball to release for standalone builds

With the advent of https://reviews.llvm.org/D131919 and
a11cd0d94e
 the third-party directory is required to build LLVM and other packages and in standalone
builds the third-party directory is not available from the llvm tarball anymore.

Differential Revision: https://reviews.llvm.org/D137777

llvm/utils/release/export.sh

@@ -13,7 +13,7 @@
 
 set -e
 
-projects="llvm clang cmake compiler-rt libcxx libcxxabi libclc clang-tools-extra polly lldb lld openmp libunwind flang"
+projects="llvm clang cmake compiler-rt libcxx libcxxabi libclc clang-tools-extra polly lldb lld openmp libunwind flang third-party"
 
 release=""
 rc=""