Fix building RelWithDebInfo due to undeclared identifier isnan

[DarlCat]

Simple swap of isnan to llisnan to fix building RelWithDebinfo on Linux. Specifically with Clang 22.1.3

Contributed with permission and blessing from @XenHat

indra/llinventory/llsettingsbase.cpp:767:15: error: use of undeclared identifier 'isnan'
  767 |     llassert(!isnan(blendf));
      |               ^~~~~

[DarlCat]

Added note: this is going to conflict with linden's develop-linux branch anyway, since that has a std::isnan() pass which is currently missing in FS. Felt a code tag may just be more disruptive to that than the conflict itself.