Add support for OpenBSD - #13

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x5176b2]

goroutine 12 [running]:
github.com/struCoder/pidusage.GetStat(...)
	/home/build/pidusage/pidusage.go:193
github.com/struCoder/pidusage.BenchmarkGetStat(0xc0000b4240)
	/home/build/pidusage/pidusage_test.go:12 +0x72
testing.(*B).runN(0xc0000b4240, 0x1)
	/usr/local/go/src/testing/benchmark.go:192 +0xeb
testing.(*B).run1.func1(0xc0000b4240)
	/usr/local/go/src/testing/benchmark.go:232 +0x57
created by testing.(*B).run1
	/usr/local/go/src/testing/benchmark.go:225 +0x7f
exit status 2
FAIL	github.com/struCoder/pidusage	0.098s

go test -bench Get   
goos: openbsd
goarch: amd64
pkg: github.com/struCoder/pidusage
cpu: Intel(R) Core(TM) i5-5250U CPU @ 1.60GHz
BenchmarkGetStat 	  258110	      4490 ns/op
PASS
ok  	github.com/struCoder/pidusage	1.229s