talloc: Make it compile with MSVC, MinGW, and Xcode/gcc4.0.

Based on Aras Pranckevičius' patch.
diff --git a/src/SConscript b/src/SConscript
index 2b46186..c3e34be 100644
--- a/src/SConscript
+++ b/src/SConscript
@@ -5,6 +5,9 @@
     SConscript('egl/main/SConscript')
 
 if 'mesa' in env['statetrackers']:
+    if platform == 'windows':
+        SConscript('talloc/SConscript')
+
     SConscript('glsl/SConscript')
     SConscript('mapi/glapi/SConscript')
     SConscript('mesa/SConscript')