[NFC] Move the export attribute after extern "C".

Not all compilers support attributes before `extern "C"`. gcc is the main one
that doesn't support it.

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@358301 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed