[lib/utils] fix include for waitpid()
[lib/utils] fix include for waitpid()

The POSIX include for waitpid() is sys/wait.h, so use that instead of
the unportable wait.h (which was excluded on Mac as workaround).

This should make this file build on any POSIX platform, and not just
only on Linux and Mac.

KDE/qtcurve,KDE/qtcurve,KDE/qtcurve,KDE/qtcurve