Add missing STBIRDEF to allow multiple stb_image_resize2.h implementations

[rcurtin]

The following code that includes the STB image resize implementation in multiple files will fail:

a.cpp:

#define STB_IMAGE_RESIZE_IMPLEMENTATION
#define STB_IMAGE_RESIZE_STATIC
#include "stb_image_resize2.h"

int a()
{
  return 3;
}

b.cpp:

#define STB_IMAGE_RESIZE_IMPLEMENTATION
#define STB_IMAGE_RESIZE_STATIC
#include "stb_image_resize2.h"

int b()
{
  return 2;
}

main.cpp:

int a();
int b();

int main()
{
  return a() + b();
}

Compile with

g++ -o main main.cpp a.cpp b.cpp

and you get

/usr/bin/ld: /tmp/cciFeflx.o:(.data+0x420): multiple definition of `stbir__s32_32768'; /tmp/cctlLRFu.o:(.data+0x420): first defined here
/usr/bin/ld: /tmp/cciFeflx.o:(.data+0x430): multiple definition of `stbir__s16_32768'; /tmp/cctlLRFu.o:(.data+0x430): first defined here
collect2: error: ld returned 1 exit status

The fix is easy: just a missing STBIRDEF in front of some simd constant variables, so that they are defined with static and can be included in multiple translation units if STB_IMAGE_RESIZE_STATIC is defined. Once the fix in this PR is applied, the test program links happily.

Thanks so much for STB! We use it inside of mlpack to provide image support for machine learning tasks.

[jeffrbig2]

Dang, thought I had them all - I'll probably do this on my end, since I need to do it for all the different SIMD versions...