Managed to find out a solution by myself.

• removed the three #define NO_PORTx_PINCHANGES

• removed the #include <PinChangeInt.h>

• changed PCintPort::attachInterrupt(pinInclusion, startInclusionInterrupt, RISING); to
  attachInterrupt(1, startInclusionInterrupt, RISING);

Now I am able to use the Seeedstudip GPRS shield pins 7 and 8 with SoftwareSerial.