crt: Use (void*) casts to silence warnings about function pointer casts

Modern GCC and Clang warn for these casts, with warnings like these:

../secapi/_chsize_s.c:20:6: warning: cast from 'FARPROC' (aka 'long long (*)()') to 'errno_t ((*))(int, long long) __attribute__((cdecl))' (aka 'int (*)(int, lo
ng long)') converts to incompatible function type [-Wcast-function-type-mismatch]
   20 |         f = (errno_t __cdecl (*)(int, long long))
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   21 |             GetProcAddress (__mingw_get_msvcrt_handle (), "_chsize_s");
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

By casting to (void*) instead, we can let the compiler do the second
cast to the target function type implicitly, and also silencing these
warnings.

Signed-off-by: Martin Storsjö <martin@martin.st>
diff --git a/mingw-w64-crt/secapi/memcpy_s.c b/mingw-w64-crt/secapi/memcpy_s.c
index 013eff0..a206de3 100644
--- a/mingw-w64-crt/secapi/memcpy_s.c
+++ b/mingw-w64-crt/secapi/memcpy_s.c
@@ -16,7 +16,7 @@
 
   if (f == _stub)
     {
-	f = (errno_t __cdecl (*)(void *, size_t, const void *, size_t))
+	f = (void*)
 	    GetProcAddress (__mingw_get_msvcrt_handle (), "memcpy_s");
 	if (!f)
 	  f = _int_memcpy_s;