lib/tests/igt_fork.c: Fix error in mmap() flags

In subtest_leak(), mmap() is called with the flag PROT_WRITE,
but no PROT_READ. Later in the function, the mapped memory is
read using `children[i]`. In FreeBSD, the lack of PROT_READ
causes SIGSEGV. Adding the PROT_READ flag to the mmap() call
fixes this.

Signed-off-by: Jake Freeland <jfree@freebsd.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>