include/devname.h: add missing header

The devname.h requires readproc.h header which has dev_t. Unless
header is included the includes in program files must be in
correct order. I also added FIXME item to remind that for uid &
gid value int not correct data type but uid_t or gid_t from
pwd.h.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

proc/devname.h

@@ -2,6 +2,7 @@
 #define PROC_DEVNAME_H
 
 #include "procps.h"
+#include "readproc.h"
 
 EXTERN_C_BEGIN