Fix Windows build.
Fix Windows build.

* build.conf
  (libsvn_subr): add svn_mutex.h to exports

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion