gdb.rocm: Don't include unistd.h when not necessary

gdb.rocm/code-object-load-while-breakpoint-hit.cpp is including
unistd.h unnecessarily AFAICS.  Don't include it, so this test can
compile on Windows (x86_64-pc-windows-msvc) as well.

Approved-by: Lancelot Six <lancelot.six@amd.com> (amdgpu)
Change-Id: I9f469fd4b061ab9895c8bbbd408939e4df2a4e92