testsuite: Skip T493 in ghci way.

T493 tests #493, which is an FFI test. FFI tests should be skipped
in ghci way.

testsuite/tests/ffi/should_run/all.T

@@ -207,4 +207,4 @@ test('PrimFFIInt16', [omit_ways(['ghci'])], compile_and_run, ['PrimFFIInt16_c.c'
 
 test('PrimFFIWord16', [omit_ways(['ghci'])], compile_and_run, ['PrimFFIWord16_c.c'])
 
-test('T493', [], compile_and_run, ['T493_c.c'])
+test('T493', [omit_ways(['ghci'])], compile_and_run, ['T493_c.c'])