Remove unneeded ymath.h include from int_math.h

This avoids a conflict between stdbool.h, which defines bool to _Bool in
xkeycheck.h. From what I can tell, ymath.h is an internal header, and
the intention is that users should include math.h directly instead. It
doesn't appear to provide declarations of anything required for our
builtins. This include was added back in r249513 from 2015, and it's
possible that ymath.h provided something this code needed at the time,
but today it does not.

llvm-svn: 357728