build: Use -fvisibility=hidden for plugins

cmake/modules/ImHexPlugin.cmake

@@ -46,6 +46,15 @@ macro(add_imhex_plugin)
     addIncludesFromLibrary(${IMHEX_PLUGIN_NAME} libpl)
     addIncludesFromLibrary(${IMHEX_PLUGIN_NAME} libpl-gen)
 
+    if (NOT IMHEX_PLUGIN_LIBRARY_PLUGIN)
+        set_target_properties(${IMHEX_PLUGIN_NAME}
+            PROPERTIES
+                C_VISIBILITY_PRESET hidden
+                CPP_VISIBILITY_PRESET hidden
+                VISIBILITY_INLINES_HIDDEN ON
+        )
+    endif()
+
     precompileHeaders(${IMHEX_PLUGIN_NAME} "${CMAKE_CURRENT_SOURCE_DIR}/include")
 
     # Add IMHEX_PROJECT_NAME and IMHEX_VERSION define