stb_image_write: Disable HDR writer completely in NO_STDIO build

Fixes issue #793, hopefully.

stb_image_write.h

@@ -622,6 +622,8 @@ STBIWDEF int stbi_write_tga(char const *filename, int x, int y, int comp, const
 
 #define stbiw__max(a, b)  ((a) > (b) ? (a) : (b))
 
+#ifndef STBI_WRITE_NO_STDIO
+
 static void stbiw__linear_to_rgbe(unsigned char *rgbe, float *linear)
 {
    int exponent;
@@ -777,7 +779,6 @@ STBIWDEF int stbi_write_hdr_to_func(stbi_write_func *func, void *context, int x,
    return stbi_write_hdr_core(&s, x, y, comp, (float *) data);
 }
 
-#ifndef STBI_WRITE_NO_STDIO
 STBIWDEF int stbi_write_hdr(char const *filename, int x, int y, int comp, const float *data)
 {
    stbi__write_context s = { 0 };