Add message FilerDevicePath to headers

C version of the definition from Hdr:Messages.
Retagged as RISC_OSLib-5_87.

rlib/h/wimp

@@ -331,6 +331,7 @@ typedef enum {
   wimp_MFILER_ACTION              = 0x405,
   wimp_MFILER_CONTROL_ACTION      = 0x406,
   wimp_MFILER_SELECTION           = 0x407,
+  wimp_MFILER_DEVICEPATH          = 0x408,
 
   wimp_MHELPREQUEST  = 0x502,         /* interactive help request */
   wimp_MHELPREPLY    = 0x503,         /* interactive help message */