embed/port: Fix alloca include for Windows platforms.

When building the embedded port on MinGW-w64, I receive the following
error:

    fatal error: alloca.h: No such file or directory

MinGW-w64 (used on MSYS2) doesn't include `alloca.h`, but `alloca()` is
provided via `malloc.h` instead.  And this fix is also needed for other
Windows build systems.

Signed-off-by: SiZiOUS <[email protected]>

Author: sizious

ports/embed/port/mpconfigport_common.h

@@ -34,8 +34,13 @@ typedef long mp_off_t;
 
 // Need to provide a declaration/definition of alloca()
 #if defined(__FreeBSD__) || defined(__NetBSD__)
+// BSD
 #include <stdlib.h>
+#elif defined(_WIN32)
+// Windows
+#include <malloc.h>
 #else
+// Other OS
 #include <alloca.h>
 #endif