stb_image.h: Only define stbi__l2h_{gamme,scale} when needed.

This fixes a (well, actually the only) compiler warning.

stb_image.h

@@ -1186,14 +1186,15 @@ STBIDEF int      stbi_is_hdr_from_callbacks(stbi_io_callbacks const *clbk, void
    #endif
 }
 
-static float stbi__h2l_gamma_i=1.0f/2.2f, stbi__h2l_scale_i=1.0f;
+#ifndef STBI_NO_LINEAR
 static float stbi__l2h_gamma=2.2f, stbi__l2h_scale=1.0f;
 
-#ifndef STBI_NO_LINEAR
 STBIDEF void   stbi_ldr_to_hdr_gamma(float gamma) { stbi__l2h_gamma = gamma; }
 STBIDEF void   stbi_ldr_to_hdr_scale(float scale) { stbi__l2h_scale = scale; }
 #endif
 
+static float stbi__h2l_gamma_i=1.0f/2.2f, stbi__h2l_scale_i=1.0f;
+
 STBIDEF void   stbi_hdr_to_ldr_gamma(float gamma) { stbi__h2l_gamma_i = 1/gamma; }
 STBIDEF void   stbi_hdr_to_ldr_scale(float scale) { stbi__h2l_scale_i = 1/scale; }