Fix compiler warning for type of luai_ipow()

The Lua VM's luai_ipow() function is reused in lmathlib to provide
an integer version of the pow() function. However luai_ipow's
declaration in luac is only for LUA_CORE, whereas lmathlib.c is LUA_LIB.
The least invasive fix is just to redeclare it in lmathlib.c