Fixup: Re-add -fPIC

Fixup of c00fdb1

The mentioned commit removes the `-fPIC` flag, but this is needed in
Zyn: Even though zyn links rtosc as a `.a` file, the plugin versions are
shared objects and do require that every part, including rtosc is
compiled with `-fPIC`.

On the other side it is desired to compile rtosc as a `.a` into zyn to
be able to use link time optimization (LTO) and for distributional
reasons.

CMakeLists.txt

@@ -119,6 +119,8 @@ target_compile_features(rtosc-cpp PUBLIC cxx_std_11)
 target_include_directories(rtosc  PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/include)
 target_compile_options(rtosc      PRIVATE
         "$<$<NOT:$<CXX_COMPILER_ID:MSVC>>:-std=c99>")
+set_target_properties(rtosc PROPERTIES POSITION_INDEPENDENT_CODE ON)
+set_target_properties(rtosc-cpp PROPERTIES POSITION_INDEPENDENT_CODE ON)
 
 if(RTOSC_IWYU_PATH AND RTOSC_INCLUDE_WHAT_YOU_USE)
     set(RTOSC_IWYU_PATH_AND_OPTIONS