Install mimalloc headers in sysroot

[Markus87]

In my tests the mimalloc headers were available after this change.
This should resolve #21652.

Should you need a test for this I will do my best to provide one.

[sbc100]

We should probably add a small test (to test_other.py) that shows that we can use these headers in a program.

[Markus87]

@sbc100 Would that be enough? If it is I will add it to this pull request.

emsdk/upstream/emscripten$ python test/runner.py other.test_mimalloc_headers

  @with_both_compilers
  def test_mimalloc_headers(self, compiler):
    src = r'''
      #include <mimalloc.h>

      int main() {
          mi_option_enable(mi_option_verbose);
          return 0;
      }
    '''
    self.do_run(src, emcc_args=['-sMALLOC=mimalloc'] )

It runs with the changes of this pull request, if I remove the changes to system_libs.py and run emcc --clear-cache it fails.

[sbc100]

@sbc100 Would that be enough? If it is I will add it to this pull request.

emsdk/upstream/emscripten$ python test/runner.py other.test_mimalloc_headers

  @with_both_compilers
  def test_mimalloc_headers(self, compiler):
    src = r'''
      #include <mimalloc.h>

      int main() {
          mi_option_enable(mi_option_verbose);
          return 0;
      }
    '''
    self.do_run(src, emcc_args=['-sMALLOC=mimalloc'] )

It runs with the changes of this pull request, if I remove the changes to system_libs.py and run emcc --clear-cache it fails.

Perfect. (You don't need with_both_compilers there though).