Escape spaces in depfile with backslashes.

This matches how Linux escapes spaces in paths.
The same syntax is also used by other build tools that output depfiles,
e.g. edd36eba5e/src/cargo/core/compiler/output_depinfo.rs (L19)

Signed-off-by: Colin Finck <mail@colinfinck.de>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

dtc.c

@@ -289,7 +289,9 @@ int main(int argc, char *argv[])
 		if (!depfile)
 			die("Couldn't open dependency file %s: %s\n", depname,
 			    strerror(errno));
-		fprintf(depfile, "%s:", outname);
+
+		fprint_path_escaped(depfile, outname);
+		fputc(':', depfile);
 	}
 
 	if (inform == NULL)