Update squish library to compile under Linux

There is a fix in the libsquish repository to make v1.11 compile under
Linux.  http://code.google.com/p/libsquish/  This involves adding an
#include <limits.h> to a couple of files.

Engine/lib/squish/alpha.cpp

@@ -25,6 +25,7 @@
    
 #include "alpha.h"
 #include <algorithm>
+#include <limits.h> 
 
 namespace squish {
 

Engine/lib/squish/singlecolourfit.cpp

@@ -26,6 +26,7 @@
 #include "singlecolourfit.h"
 #include "colourset.h"
 #include "colourblock.h"
+#include <limits.h> 
 
 namespace squish {