cmake: Fix PLATFORM_WEB build

Did this ever work? Surely, doesn't look like it...

src/textures.c

@@ -568,6 +568,7 @@ void ExportImage(const char *fileName, Image image)
     // NOTE: Getting Color array as RGBA unsigned char values
     unsigned char *imgData = (unsigned char *)GetImageData(image);
     SavePNG(fileName, imgData, image.width, image.height, 4);
+    // FIXME ^ this fails on PLATFORM_WEB, what should we do?
     free(imgData);
 }