Try to fix compilation error about the new threads default

lib/Slic3r/Config.pm

@@ -218,7 +218,7 @@ sub validate {
     my $self = shift;
     
     # -j, --threads
-    die "Invalid value for --threads\n"
+    die "Invalid value for --threads: " . $self->threads . "\n"
         if $self->threads < 1;
 
     # --layer-height

xs/src/libslic3r/PrintConfig.cpp

@@ -1293,7 +1293,7 @@ PrintConfigDef::PrintConfigDef()
     def->min = 1;
     def->max = 16;
     {
-        int threads = boost::thread::hardware_concurrency();
+        unsigned int threads = boost::thread::hardware_concurrency();
         if (threads == 0) threads = 2;
         def->default_value = new ConfigOptionInt(threads);
     }