Don't include the colon in /Fo for MSVC

Closes #66
diff --git a/src/lib.rs b/src/lib.rs
index a15376e..f3bf669 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -352,7 +352,7 @@
         if msvc && is_asm {
             cmd.arg("/Fo").arg(dst);
         } else if msvc {
-            let mut s = OsString::from("/Fo:");
+            let mut s = OsString::from("/Fo");
             s.push(&dst);
             cmd.arg(s);
         } else {