Build error on FreeBSD

[rastislavs]

This build error is from devel branch, but the same issue is present also in master:

[ 2%] Building C object CMakeFiles/yang.dir/src/common.c.o
/home/rasto/tmp/libyang/src/common.c:54:17: error: use of undeclared identifier 'ly_err_main'
if (ptr != &ly_err_main) {
^
1 error generated.
*** Error code 1

The content of ly_err_free function should be probably wrapped into #ifdef linux.