Add sensors to external. (#2820)

Author: floitsch

external/CMakeLists.txt

@@ -84,6 +84,7 @@ set(TOIT_REPOS
     https://github.com/toitware/toit-r46ca01
     https://github.com/toitware/toit-resistance-to-temperature
     https://github.com/toitware/toit-rs485
+    https://github.com/toitware/toit-sensors
     https://github.com/toitware/toit-si7006
     https://github.com/toitware/toit-solar-position
     https://github.com/toitware/toit-ssd1306