Make the dummy method public.

This may satisfy the Watcom compiler:

 D:\Dashboards\My Tests\CMake\Tests\PositionIndependentTargets\pic_lib.cpp(7): Warning! W011: col(1) all members are private
 Linking CXX static library test_target_static_library_properties.lib
 [ 53%] Built target test_target_static_library_properties
 Scanning dependencies of target sharedlib
 [ 61%] Building CXX object interface/CMakeFiles/sharedlib.dir/__/pic_lib.cpp.obj
 Linking CXX shared library sharedlib.dll
 Error! No exports in 'sharedlib.dll'
 Error(E42): Last command making (interface\sharedlib.dll) returned a bad status
 Error(E02): Make execution terminated
 Error(E42): Last command making (interface\CMakeFiles\sharedlib.dir\all) returned a bad status
 Error(E02): Make execution terminated
 Error(E42): Last command making (all) returned a bad status
 Error(E02): Make execution terminated
Merge topic 'INTERFACE_LIBRARY-target-type' into next

fb8329d Make the dummy method public.