check_disk is missing -lrt on Solaris (#1329)

The following is being reported against monitoring-plugins-v2.1.1-39-g8095.

In the link of `check_disk.o` on Solaris 8, I get:

    Undefined                       first referenced
    symbol                             in file
    nanosleep                           check_disk.o
    ld: fatal: Symbol referencing errors. No output written to check_disk
    *** Error code 1
    make: Fatal error: Command failed for target `check_disk'

A look at the man page for nanosleep(3RT) on this platform shows that
it requires the `-lrt` library to be linked in, to access this function.


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