pintos: Avoid apparent name collision in definition of SIGALRM subroutine.

Without this patch, modern versions of Perl issue the following warnings:

    Prototype mismatch: sub main::SIGVTALRM () vs none at
    .../src/utils/pintos line 188.
    Constant subroutine SIGVTALRM redefined at
    .../src/utils/pintos line 180.

Changing the name of the function avoids the warning.

Thanks to Geoffrey Knopf for reporting this problem.