test-math-iscanonical.cc: Replace bool with int

Fix GCC 7 compilation error:

test-math-iscanonical.cc: In function ‘void check_type()’:
test-math-iscanonical.cc:33:11: error: use of an operand of type ‘bool’ in ‘operator++’ is deprecated [-Werror=deprecated]
     errors++;
           ^~

	* math/test-math-iscanonical.cc (error): Replace bool with int.