Add missing rust type

[equation314]

#431 added basic rust support, but some types are missing. Debugging the rust program may cause pwndbg to throw the following exception:

Exception occured: Error: No type named int. (<class 'gdb.error'>)
For more info invoke `set exception-verbose on` and rerun the command
or debug it by yourself with `set exception-debugger on`
Python Exception <class 'gdb.error'> No type named int.: 

[disconnect3d]

Thanks for the PR! It looks good and I assume it works fine :P.

JFYI: The solution we use is not that good in the long term.We should probably handle show language and set types based on that.
Pwndbg fails also with Go binaries in as imilar manner; I played with it once and I had some problems to make it work...