ICU-20566 Add missing ICU namespace macros for extra files

ICU-20566 Add missing ICU namespace macros for extra files

ICU now uses namespaces. When trying to use the scrptrun.h header, the
header cannot compile stand-alone as it is inheriting from UObject which
is namespaced now. Add namespace macros to properly inherit.

ICU-20566 Add scrptrun to the extra high level Makefile

The scrptrun project was not being added to the extra portion of the ICU
build. Add it as a sub project so it can always be built.

ICU-20566 Properly namespace the ScriptRun test

The ScriptRun class is now namespaced.  Update the test reference.

ICU-20566 Use int instead of void to compile

Clang will complain about having a void main without an int main to
invoke. Change void to int and return 0.