2.6.x - core_esp8266_features.h error: unknown type name 'uint32_t'

[Links2004]

latest master and 2.6.1 are not compiling when "core_esp8266_features.h" is used.

/home/travis/arduino_ide/hardware/esp8266com/esp8266/cores/esp8266/core_esp8266_features.h:88:1: error: unknown type name 'uint32_t'
 inline uint32_t esp_get_cycle_count() __attribute__((always_inline));
 ^
/home/travis/arduino_ide/hardware/esp8266com/esp8266/cores/esp8266/core_esp8266_features.h:89:1: error: unknown type name 'uint32_t'
 inline uint32_t esp_get_cycle_count() {
 ^
/home/travis/arduino_ide/hardware/esp8266com/esp8266/cores/esp8266/core_esp8266_features.h: In function 'esp_get_cycle_count':
/home/travis/arduino_ide/hardware/esp8266com/esp8266/cores/esp8266/core_esp8266_features.h:90:3: error: unknown type name 'uint32_t'
   uint32_t ccount;
   ^
/home/travis/arduino_ide/hardware/esp8266com/esp8266/cores/esp8266/core_esp8266_features.h: At top level:
/home/travis/arduino_ide/hardware/esp8266com/esp8266/cores/esp8266/core_esp8266_features.h:112:24: error: unknown type name 'uint32_t'
 void precache(void *f, uint32_t bytes);
                        ^
Error compiling.
Build failed (arduino) 

full log:
https://travis-ci.org/Links2004/arduinoWebSockets/jobs/611496201

based in the build logs of travis this problem where introduced ~3 months ago.
https://travis-ci.org/Links2004/arduinoWebSockets/builds