Add drmGetDeviceNameFromFd function

Determines the /dev filename of the drm fd argument.
diff --git a/xf86drm.h b/xf86drm.h
index cf7d132..f0dfb88 100644
--- a/xf86drm.h
+++ b/xf86drm.h
@@ -708,4 +708,6 @@
 
 extern int drmHandleEvent(int fd, drmEventContextPtr evctx);
 
+extern char *drmGetDeviceNameFromFd(int fd);
+
 #endif