gpsd: correct explicit nmea 0183 support flag

The following handles the GPSd SCons change for the explicit
configuration of the NMEA 0183 protocol. The protocol configuration
change was introduced in 3.12. Developer's who transitioned to
Buildroot's GPSd 3.15 had NMEA 0183 implicitly enabled by default. This
change allows NMEA 0183 to be explicitly disabled again.

See:
 http://git.savannah.gnu.org/cgit/gpsd.git/commit/SConstruct?id=8f20d7b2d1ece8f3205ca038726c77daa5234c0c



Signed-off-by: James Knight <james.knight@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>