Use correct extension name.

Like 0068c7bb25, but for other shaders, too

files/shaders/groundcover_fragment.glsl

@@ -1,7 +1,7 @@
 #version 120
 
 #if @useGPUShader4
-    #extension EXT_gpu_shader4: require
+    #extension GL_EXT_gpu_shader4: require
 #endif
 
 #define GROUNDCOVER

files/shaders/shadowcasting_fragment.glsl

@@ -1,7 +1,7 @@
 #version 120
 
 #if @useGPUShader4
-    #extension EXT_gpu_shader4: require
+    #extension GL_EXT_gpu_shader4: require
 #endif
 
 uniform sampler2D diffuseMap;