Fix 35_SignedDistanceFieldText sample for OpenGL ES 2.0

Thanks elix22 https://github.com/urho3d/Urho3D/pull/2544

bin/CoreData/Shaders/GLSL/Text.glsl

@@ -1,3 +1,6 @@
+// https://www.khronos.org/registry/OpenGL/extensions/OES/OES_standard_derivatives.txt
+#extension GL_OES_standard_derivatives : enable
+
 #include "Uniforms.glsl"
 #include "Samplers.glsl"
 #include "Transform.glsl"