Fix C++ signature of function mi_is_in_heap_region
Fix C++ signature of function mi_is_in_heap_region

Original code did not compile with MI_USE_CXX=ON as function
mi_is_in_heap_region was declared noexcept through macro mi_attr_noexcept
but defined without specification, i.e. implicitly with noexception(false)

Just adding the macro in the definition is enough to fix the issue
(tested with gcc 8.3.5)

microsoft/mimalloc,microsoft/mimalloc