sttb_image: Unused functions when STBI_NO_JPEG is defined

[BlackMightyRavenDark]

When defining STBI_NO_JPEG, the GCC compiler starts to show warnings:

stb_image.h:1078:12: warning: 'int stbi__mul2shorts_valid(int, int)' defined but not used [-Wunused-function] 1078 | static int stbi__mul2shorts_valid(int a, int b) stb_image.h:1070:12: warning: 'int stbi__addints_valid(int, int)' defined but not used [-Wunused-function] 1070 | static int stbi__addints_valid(int a, int b)

So, these functions must be wrapped into #ifndef STBI_NO_JPEG (maybe something else).