Make Greeter link privately to fmt (#109)

Greeter depends on fmt only in it's implementation file, therefore this
dependency can be private.

Author: hazelnusse

CMakeLists.txt

@@ -52,7 +52,7 @@ set_target_properties(Greeter PROPERTIES CXX_STANDARD 17)
 target_compile_options(Greeter PUBLIC "$<$<COMPILE_LANG_AND_ID:CXX,MSVC>:/permissive>")
 
 # Link dependencies
-target_link_libraries(Greeter PUBLIC fmt::fmt)
+target_link_libraries(Greeter PRIVATE fmt::fmt)
 
 target_include_directories(
   Greeter PUBLIC $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>