warning: `munit_plus_error_jmp` switches from `class` to `struct`

[codylico]

from AppVeyor:

munit.cpp(1468): warning C4099: '`anonymous-namespace'::munit_plus_error_jmp': type name first seen using 'class' now seen using 'struct'
munit.cpp(157): note: see declaration of '`anonymous-namespace'::munit_plus_error_jmp'