/home/berni/Arduino/libraries/Adafruit_SSD1306-master/Adafruit_SSD1306.h:49:26: fatal error: Adafruit_GFX.h: No such file or directory
#include <Adafruit_GFX.h>
The error message quite clearly shows that the include is in the file "Adafruit_SSD1306.h", not in the code you posted.