Guard folly headers when NETWORKX=OFF

[siyuan0322]

What do these changes do?

Fix the build error when NETWORKX=OFF
Remove a warning.

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Merging #1259 (0493877) into main (a45a7b9) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1259      +/-   ##
==========================================
- Coverage   73.71%   73.70%   -0.02%     
==========================================
  Files          89       89              
  Lines        9474     9474              
==========================================
- Hits         6984     6983       -1     
- Misses       2490     2491       +1     

Impacted Files	Coverage Δ
python/graphscope/client/rpc.py	85.12% <0.00%> (-0.83%)	⬇️

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