Fix isnan already defined
Fix isnan already defined

In recent vs15 compailer isnan function is already defined, so I wrapped
the definition with #ifndef

rednaga/yara,kallanreed/yara,plusvic/yara,MeteorAdminz/yara,VirusTotal/yara,cblichmann/yara,plusvic/yara,tanium/yara,VirusTotal/yara,hillu/yara,kallanreed/yara,pombredanne/yara,pombredanne/yara,pombredanne/yara,pombredanne/yara,tanium/yara,wxsBSD/yara,VirusTotal/yara,wxsBSD/yara,wxsBSD/yara,cblichmann/yara,rednaga/yara,plusvic/yara,cblichmann/yara,MeteorAdminz/yara,jpypi/yara,wxsBSD/yara,cblichmann/yara,hillu/yara,pombredanne/yara,jpypi/yara,VirusTotal/yara,VirusTotal/yara,hillu/yara,plusvic/yara,hillu/yara,plusvic/yara,kallanreed/yara,jpypi/yara,cblichmann/yara,tanium/yara,MeteorAdminz/yara,hillu/yara,wxsBSD/yara,rednaga/yara