add pRoloc to Suggests

Dear Laurent,

`R CMD check MSnbase_1.11.10tar.gz` fails if `pRoloc` is not installed:

    * checking running R code from vignettes ...
       ‘MSnbase-demo.Rnw’ ... OK
       ‘MSnbase-development.Rnw’ ... OK
       ‘MSnbase-io.Rnw’ ... failed
     ERROR
    Errors in running code in vignettes:
    when running code in ‘MSnbase-io.Rnw’
      ...
    3                      unknown    unknown
    
    > f1 <- dir(system.file("extdata", package = "pRoloc"), 
    +     full.names = TRUE, pattern = "exprsFile.csv")
    
    > exprsCsv <- read.csv(f1)
    
      When sourcing ‘MSnbase-io.R’:
    Error: invalid 'description' argument
    Execution halted

To reproduce:
```s
remove.packages("pRoloc")
```
```bash
R CMD check MSnbase_1.11.10.tar.gz
```

Best wishes,

Sebastian