Remove unnecessary single quoting of debugstr_xxx strings.

diff --git a/files/drive.c b/files/drive.c
index 9b0d169..8e790e4 100644
--- a/files/drive.c
+++ b/files/drive.c
@@ -1206,7 +1206,7 @@
     {
         if ((root[1]) && (root[1] != ':'))
 	{
-	    WARN("invalid root '%s'\n", debugstr_a(root));
+	    WARN("invalid root %s\n", debugstr_a(root));
 	    return DRIVE_NO_ROOT_DIR;
 	}
 	drive = toupper(root[0]) - 'A';