systemc: Include cstring in the tlm header file.

A lot of TLM implementation assumes the string functions (memcpy,
memset, etc) are available, but don't include cstring. Lets do that
for them to avoid having to fix up a lot of places.

Change-Id: I1f26d3a227fe2cefc2613e24cbb0333216e46ee9
Reviewed-on: https://gem5-review.googlesource.com/c/15060
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>

src/systemc/ext/tlm

@@ -20,6 +20,7 @@
 #ifndef __SYSTEMC_EXT_TLM__
 #define __SYSTEMC_EXT_TLM__
 
+#include <cstring>
 #include <systemc>    // main SystemC header
 
 #include "tlm_core/2/version.h"