headers: Add missing declaration for __mingw_aligned_offset_malloc()

There is already declaration for msvcrt _aligned_offset_malloc() function,
so add export also declaration for mingw __mingw_aligned_offset_malloc()
function.

At the same time, remove also uneeded declaration of this function from
mingw-w64-crt/misc/mingw-aligned-malloc.c file and use the new one from
malloc.h.

Signed-off-by: Martin Storsjö <martin@martin.st>