escape spaces with backslash when writing dep file

filenames are sparated by spaces in the dep file.  if a filename in the
dep file contains spaces they must be escaped, otherwise the tool that
reads the dep file will see multiple wrong filenames.

kernel/yosys.cc

@@ -482,6 +482,20 @@ void remove_directory(std::string dirname)
 #endif
 }
 
+std::string escape_filename_spaces (const std::string& filename)
+{
+  std::string out;
+  out.reserve (filename.size ());
+  for (auto c : filename)
+  {
+    if (c == ' ')
+      out += "\\ ";
+    else
+      out.push_back (c);
+  }
+  return out;
+}
+
 int GetSize(RTLIL::Wire *wire)
 {
 	return wire->width;