Fix win7 signature

Author: ChrisDenton

src/symbolize/dbghelp.rs

@@ -128,7 +128,7 @@ unsafe fn resolve_legacy(
     addr: *mut c_void,
     _inline_context: Option<DWORD>,
     cb: &mut dyn FnMut(&super::Symbol),
-) {
+) -> Option<()> {
     let addr = super::adjust_ip(addr) as DWORD64;
     do_resolve(
         |info| dbghelp.SymFromAddrW()(GetCurrentProcess(), addr, &mut 0, info),