xf86drm: Be sure to closedir before return

removed in commit bb45ce4e3a

Adding it back as it is still needed in the case where we don't find a
match.

Signed-off-by: Kevin Strasser <kevin.strasser@intel.com>
Fixes: bb45ce4e3a "libdrm: Use readdir instead of readdir_r to
                             avoid build warnings"$
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

xf86drm.c

@@ -2855,6 +2855,8 @@ static char *drmGetMinorNameForFD(int fd, int type)
             return strdup(dev_name);
         }
     }
+
+    closedir(sysdir);
     return NULL;
 #else
     struct stat sbuf;