Compile with `-fvisibility=hidden`.

Fixes errors when spdlog is statically linked into two shared libraries, caused by the one copy calling a function of the other. This will not cause any problems for consumers because the exported APIs are already marked with `SPDLOG_API`.

CMakeLists.txt

@@ -35,6 +35,7 @@ endif ()
 
 
 set(CMAKE_CXX_EXTENSIONS OFF)
+set(CMAKE_CXX_VISIBILITY_PRESET hidden)
 
 if (CMAKE_SYSTEM_NAME MATCHES "CYGWIN" OR CMAKE_SYSTEM_NAME MATCHES "MSYS" OR CMAKE_SYSTEM_NAME MATCHES "MINGW")
     set(CMAKE_CXX_EXTENSIONS ON)