MSVC: can't use `malloc()` in `string.c` with `WIN32_LEAN_AND_MEAN`

[kou]

https://github.com/kou/mruby/actions/runs/9278121120/job/25528601313#step:4:75

string.c
D:\a\mruby\mruby\src\string.c(859): error C4013: 'malloc' undefined; assuming extern returning int
D:\a\mruby\mruby\src\string.c(859): warning C4312: 'type cast': conversion from 'int' to 'wchar_t *' of greater size
D:\a\mruby\mruby\src\string.c(866): warning C4312: 'type cast': conversion from 'int' to 'char *' of greater size
D:\a\mruby\mruby\src\string.c(868): error C4013: 'free' undefined; assuming extern returning int
D:\a\mruby\mruby\src\string.c(889): warning C4312: 'type cast': conversion from 'int' to 'wchar_t *' of greater size
D:\a\mruby\mruby\src\string.c(895): warning C4312: 'type cast': conversion from 'int' to 'char *' of greater size

If WIN32_LEAN_AND_MEAN is defined, windows.h doesn't include additional headers.
See also: https://learn.microsoft.com/en-us/windows/win32/winprog/using-the-windows-headers#faster-builds-with-smaller-header-files

It seems that stdlib.h and malloc.h are also excluded with WIN32_LEAN_AND_MEAN. They are required to use malloc().
https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/malloc?view=msvc-170#requirements

Routine	Required header
malloc	<stdlib.h> and <malloc.h>

We can choose either of the following solutions:

1. Include <stdlib.h> and <malloc.h> in src/string.c
2. Use mrb_malloc()/mrb_free() instead of raw malloc()/free() for mrb_locale_from_utf8()/mrb_utf8_from_locale()

1. is easy because we can just add 2 lines. But 2. may be better to use our allocation functions in our code base. But 2. breaks backward compatibility.

[kou]

#6278 uses the 2. approach.

[kou]

#6279 uses the 1. approach.

[matz]

I chose #6279