Restrict `GL_TEXTURE_EXTERNAL_OES` to Android platform

drivers/gles2/rasterizer_storage_gles2.cpp

@@ -565,7 +565,11 @@ void RasterizerStorageGLES2::texture_allocate(RID p_texture, int p_width, int p_
 			texture->images.resize(1);
 		} break;
 		case VS::TEXTURE_TYPE_EXTERNAL: {
+#ifdef ANDROID_ENABLED
 			texture->target = _GL_TEXTURE_EXTERNAL_OES;
+#else
+			texture->target = GL_TEXTURE_2D;
+#endif
 			texture->images.resize(0);
 		} break;
 		case VS::TEXTURE_TYPE_CUBEMAP: {

drivers/gles3/rasterizer_storage_gles3.cpp

@@ -666,7 +666,11 @@ void RasterizerStorageGLES3::texture_allocate(RID p_texture, int p_width, int p_
 			texture->images.resize(1);
 		} break;
 		case VS::TEXTURE_TYPE_EXTERNAL: {
+#ifdef ANDROID_ENABLED
 			texture->target = _GL_TEXTURE_EXTERNAL_OES;
+#else
+			texture->target = GL_TEXTURE_2D;
+#endif
 			texture->images.resize(0);
 		} break;
 		case VS::TEXTURE_TYPE_CUBEMAP: {