ticket:10184 Remove cplusplus ifdefs from umutex.h

git-svn-id: http://source.icu-project.org/repos/icu/icu/branches/andy@34027 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10184 Remove cplusplus ifdefs from umutex.h

git-svn-id: http://source.icu-project.org/repos/icu/icu/branches/andy@34028 251d0590-4201-4cf1-90de-194747b24ca1