runtime: don't define libc_getpid in os3_solaris.go

The function is already defined between syscall_solaris.go and
syscall2_solaris.go.

Change-Id: I034baf7c8531566bebfdbc5a4061352cbcc31449
Reviewed-on: https://go-review.googlesource.com/12773


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>