CMake: Use "hidden" symbol visibility by default

Arrange passing -fvisibility=hidden to the compiler where supported,
which effectively disables exporting symbols which are not explicitly
marked as visible.

No further changes are needed because the public API functions were
already marked with WPE_EXPORT which marks them as exported.

(cherry picked from commit baabb08)

Author: aperezdc

CMakeLists.txt

@@ -97,6 +97,8 @@ target_compile_options(wpe PRIVATE
 target_link_libraries(wpe PRIVATE XkbCommon::libxkbcommon ${CMAKE_DL_LIBS})
 
 set_target_properties(wpe PROPERTIES
+  C_VISIBILITY_PRESET hidden
+  CXX_VISIBILITY_PRESET hidden
   OUTPUT_NAME wpe-${WPE_API_VERSION}
   VERSION ${LIBWPE_VERSION}
   SOVERSION ${LIBWPE_VERSION_MAJOR}