Fix pyflakes warnings in gps-log for wildcard imports and unused modules

contrib/gps-log:7: 'datetime.datetime' imported but unused
contrib/gps-log:10: 'from gps import *' used; unable to detect undefined names
contrib/gps-log:15: gps may be undefined, or defined from star imports: gps
contrib/gps-log:15: WATCH_ENABLE may be undefined, or defined from star imports: gps
contrib/gps-log:30: isotime may be undefined, or defined from star imports: gps
contrib/gps-log:46: isnan may be undefined, or defined from star imports: gps