build failure with -DIQTREE_FLAGS=single

[reneeotten]

when trying to compile iqtree2 (both from master or the v1.2.3 tag) without omp I get the following failure:

tree/phylotree.cpp:3366:25: error: use of undeclared identifier 'omp_get_thread_num'
        int threadNum = omp_get_thread_num();

which makes sense as it should likely be in an #ifdef _OPENMP statement; perhaps like this?

#ifdef _OPENMP
    int threads = omp_get_max_threads();
#else 
    threads = 1;

[JamesBarbetti]

The compilation issue was addressed by commit 9570c60, in May 2021.