Fix a signed-compare warning.

The silk math debug macros include a bounds check on silk_abs.
Because INT_MIN = (-INT_MAX - 1), abs(INT_MIN) can't be
represented as an int. The macro was checking for this value
as 0x8000... without a cast to signed, warning on gcc.

silk/typedef.h already defines minimum values for the int
types, so we correct the warning by using those.