Fix build failures on macOS in netcpu_osx.c

[ryandesign]

netperf 2.7.0 fails to build on macOS 10.15 or later with Xcode 12 or later because as of Xcode 12 implicit declaration of function is an error. (The compiler behaves by default as if -Werror=implicit-function-declaration has been specified.) I originally reported this to MacPorts here.

After applying 0b0cbbe to fix implicit declarations of read and close, the remaining error is:

netcpu_osx.c:73:3: error: implicit declaration of function 'mach_port_deallocate' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

After fixing that by including the right header (or uncommenting the inclusion of the right header), the error is then:

netcpu_osx.c:73:37: error: too few arguments to function call, expected 2, have 1

I don't know whether the signature of mach_port_deallocate changed since this code was added or whether the code was always wrong. Mach functions are hard to find documentation for, so much so that I don't know what the second (or, it turns out, first) argument should be, but every code sample I found that used this function used mach_task_self() as the first argument, so that's what I used here. It at least compiles now.