Cleanup: silence [-Wmissing-braces] warning.

Introduced in {rBd6525e8d133b787655bdb2c2fcef218591a457c3}

Compiler: Apple LLVM version 10.0.1 (clang-1001.0.46.4)
Target: x86_64-apple-darwin18.7.0
Thread model: posix
```
source/blender/draw/engines/image/image_shader.c:46:13: warning:
suggest braces around initialization of subobject [-Wmissing-braces]
} e_data = {0}; /* Engine data */
            ^
            {}
1 warning generated.
```

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D8873

source/blender/draw/engines/image/image_shader.c

@@ -43,7 +43,7 @@ typedef struct IMAGE_Shaders {
 static struct {
   IMAGE_Shaders shaders;
   DRWShaderLibrary *lib;
-} e_data = {0}; /* Engine data */
+} e_data = {{0}}; /* Engine data */
 
 void IMAGE_shader_library_ensure(void)
 {
@@ -79,4 +79,4 @@ void IMAGE_shader_free(void)
   }
 
   DRW_SHADER_LIB_FREE_SAFE(e_data.lib);
-}
+}