remove 'register' usage

I don't think we need to add register modifier to variables right now for this
piece of code. Otherwise it has following compile warnings:

/Users/lgq/sysdig/userspace/libsinsp/filterchecks.cpp:1044:2: warning: 'register' storage class specifier is deprecated
      [-Wdeprecated-register]
        register int dt, dir;
        ^~~~~~~~~
/Users/lgq/sysdig/userspace/libsinsp/filterchecks.cpp:1044:2: warning: 'register' storage class specifier is deprecated
      [-Wdeprecated-register]
        register int dt, dir;
        ^~~~~~~~~
/Users/lgq/sysdig/userspace/libsinsp/filterchecks.cpp:1045:2: warning: 'register' storage class specifier is deprecated
      [-Wdeprecated-register]
        register struct tm *gmt, *loc;
        ^~~~~~~~~
/Users/lgq/sysdig/userspace/libsinsp/filterchecks.cpp:1045:2: warning: 'register' storage class specifier is deprecated
      [-Wdeprecated-register]
        register struct tm *gmt, *loc;
        ^~~~~~~~~