modules: make ShaderTools component dependencies explicitly empty.

The library doesn't depend on anything else from Magnum, but while for
others that's explicitly listed, here it was somehow omitted until now.

modules/FindMagnum.cmake

@@ -512,6 +512,8 @@ if(MAGNUM_TARGET_GL)
     list(APPEND _MAGNUM_Shaders_DEPENDENCIES GL)
 endif()
 
+set(_MAGNUM_ShaderTools_DEPENDENCIES )
+
 set(_MAGNUM_Text_DEPENDENCIES TextureTools)
 if(MAGNUM_TARGET_GL)
     list(APPEND _MAGNUM_Text_DEPENDENCIES GL)