CoinBronKerbosch: Replace magic number for pivoting strategy

[a-andre]

No description provided.

[christoph-cullmann]

Hi, this breaks a format string:

/local/ssd/cullmann/build/lpsolve.clpsolve/libcbc/src/CoinUtils/src/CoinBronKerbosch.cpp:375:76: error: format specifies type 'unsigned long' but the argument has underlying type 'unsigned int' [-Werror,-Wformat]
  375 |             fprintf(stderr, "Invalid option %lu for pivoting strategy!\n", pivotingStrategy_);