stb_image_write: fix clang warning

fix -Wmissing-variable-declarations clang warning

`stbi__flip_vertically_on_write` is now static like
`stbi__vertically_flip_on_load` in `stb_image.h`

stb_image_write.h

@@ -247,17 +247,17 @@ STBIWDEF void stbi_flip_vertically_on_write(int flip_boolean);
 #define STBIW_UCHAR(x) (unsigned char) ((x) & 0xff)
 
 #ifdef STB_IMAGE_WRITE_STATIC
-static int stbi__flip_vertically_on_write=0;
 static int stbi_write_png_compression_level = 8;
 static int stbi_write_tga_with_rle = 1;
 static int stbi_write_force_png_filter = -1;
 #else
 int stbi_write_png_compression_level = 8;
-int stbi__flip_vertically_on_write=0;
 int stbi_write_tga_with_rle = 1;
 int stbi_write_force_png_filter = -1;
 #endif
 
+static int stbi__flip_vertically_on_write = 0;
+
 STBIWDEF void stbi_flip_vertically_on_write(int flag)
 {
    stbi__flip_vertically_on_write = flag;