C.xs: msvc problem with MY_CXT_INIT decl

wrap it in a block as in http://perl5.git.perl.org/perl.git/commitdiff/09391cc554f6d7daf76cbe5b4c7ce18c82372b70?hp=b4fbebdebb0ca08b0fd5e052deb2d8c7d60d527a
thanks for sten for catching and fixing this