Corrected issue with SaveImageAs()

That function depends on SavePNG() and forces platform check on textures
module, that behaviour should be changed, platform definition should be
only required on core module...

src/textures.c

@@ -34,7 +34,7 @@
 *
 *   LICENSE: zlib/libpng
 *
-*   Copyright (c) 2014-2017 Ramon Santamaria (@raysan5)
+*   Copyright (c) 2014-2018 Ramon Santamaria (@raysan5)
 *
 *   This software is provided "as-is", without any express or implied warranty. In no event
 *   will the authors be held liable for any damages arising from the use of this software.
@@ -554,12 +554,14 @@ void UpdateTexture(Texture2D texture, const void *pixels)
 // Save image to a PNG file
 void SaveImageAs(const char *fileName, Image image)
 {
+#if defined(PLATFORM_DESKTOP) || defined(PLATFORM_RPI)
     // NOTE: Getting Color array as RGBA unsigned char values
     unsigned char *imgData = (unsigned char *)GetImageData(image);
     SavePNG(fileName, imgData, image.width, image.height, 4);
     free(imgData);
 
     TraceLog(LOG_INFO, "Image saved: %s", fileName);
+#endif
 }
 
 // Convert image data to desired format