Update modifier.h

added feral howl duration mod
MOD_FERAL_HOWL_DURATION = 0x1F6
Currently used in feral howl script for Monster Jackcoat +2 augmented that adds 20% duration per each merit of feral howl.
0x1F6 was already in use by something (click history on the file here on github), and you also have to increment the maximum modifier definition.