util, ext: Fix building TLM (#1105)

Fixed the issue that did not allow building TLM.

Build commands:
```bash
scons build/ARM/gem5.opt
scons setconfig build/ARM USE_SYSTEMC=n
scons --with-cxx-config --without-python --without-tcmalloc build/ARM/libgem5_opt.so
cd util/tlm
scons
```
Following this README, I tested it successfully with the simple examples:
https://gem5.googlesource.com/public/gem5/+/master/util/tlm/README

GitHub Issue: #591 
Change-Id: If07fae2eb20ad62627e733573f61bc42d594f970

---------

Co-authored-by: Ivana Mitrovic <ivanamit91@gmail.com>

ext/systemc/SConscript

@@ -25,13 +25,16 @@
 
 import os
 from m5.util.terminal import get_termcap
-import gem5_scons
+import sys
 
 Import('env')
 systemc = env.Clone()
 
 build_root = Dir('.').abspath
 src_root = Dir('.').srcdir.abspath
+gem5_root = Dir('#../..').srcnode().abspath
+sys.path.append(os.path.join(gem5_root, 'site_scons'))
+import gem5_scons
 
 systemc.Prepend(CPPPATH=Dir('./src').srcnode())
 systemc.Prepend(CPATH=Dir('./src'))