Update repositories.txt (arduino#3772)

Add geomath library, a placeholder for some geographic functions.

Author: RobTillaart

repositories.txt

@@ -1904,6 +1904,7 @@ https://github.com/RobTillaart/dateTimeHelpers
 https://github.com/RobTillaart/ellipse
 https://github.com/RobTillaart/fast_math
 https://github.com/RobTillaart/float16
+https://github.com/RobTillaart/geomath
 https://github.com/RobTillaart/infiniteAverage
 https://github.com/RobTillaart/integer24
 https://github.com/RobTillaart/map2colour