Shaders: fix setup of the tangent attribute on ES2.

Yay tests!

src/Magnum/Shaders/Phong.cpp

@@ -118,6 +118,8 @@ Phong::Phong(const Flags flags, const UnsignedInt lightCount): _flags{flags}, _l
     {
         bindAttributeLocation(Position::Location, "position");
         bindAttributeLocation(Normal::Location, "normal");
+        if(flags & Flag::NormalTexture)
+            bindAttributeLocation(Tangent::Location, "tangent");
         if(flags & (Flag::AmbientTexture|Flag::DiffuseTexture|Flag::SpecularTexture))
             bindAttributeLocation(TextureCoordinates::Location, "textureCoordinates");
     }