Add correct preprocessor check when enabling extension

files/shaders/groundcover_fragment.glsl

@@ -1,5 +1,9 @@
 #version 120
 
+#if @useGPUShader4
+    #extension EXT_gpu_shader4: require
+#endif
+
 #define GROUNDCOVER
 
 #if @diffuseMap

files/shaders/shadowcasting_fragment.glsl

@@ -1,6 +1,8 @@
 #version 120
 
-#extension EXT_gpu_shader4: enable
+#if @useGPUShader4
+    #extension EXT_gpu_shader4: require
+#endif
 
 uniform sampler2D diffuseMap;
 varying vec2 diffuseMapUV;