sim: added missed macro definition on MacOS

A recent patch add the use of the macro:
CMSG_ALIGN
This macro is not very cross-platform, and needs to be
defined according to the platform.

This patch defines the missing macro on MacOS.

Change-Id: I582f69e652dc060b4532358141179ad6d37eafc7
Reviewed-on: https://gem5-review.googlesource.com/c/16102
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>

src/sim/syscall_emul.hh

@@ -116,6 +116,10 @@
 #include "sim/syscall_emul_buf.hh"
 #include "sim/syscall_return.hh"
 
+#if defined(__APPLE__) && defined(__MACH__) && !defined(CMSG_ALIGN)
+#define CMSG_ALIGN(len) (((len) + sizeof(size_t) - 1) & ~(sizeof(size_t) - 1))
+#endif
+
 //////////////////////////////////////////////////////////////////////
 //
 // The following emulation functions are generic enough that they