Update regex match in verify_boilerplate.py

* In v0.14.0 controller-gen, we have headers like
```
//go:build !ignore_autogenerated

```
instead of
```
//go:build !ignore_autogenerated
// +build !ignore_autogenerated

```.

And this could result in error when running make verify-boilerplate
since it always expect to `//go:build !ignore_autogenerated` to not have
`\n` as next line, but followed by `// +build !ignore_autogenerated`
which contains a new line.

hack/verify/verify_boilerplate.py

@@ -121,7 +121,9 @@ def file_passes(filename, refs, regexs):  # pylint: disable=too-many-locals
         (data, found) = con.subn("", data, 1)
         con = regexs["go_build_constraints_2"]
         (data, found) = con.subn("", data, 1)
-
+        con = regexs["go_build_constraints_3"]
+        (data, found) = con.subn("", data, 1)
+
     # remove shebang from the top of shell files
     if extension in ("sh", "py"):
         she = regexs["shebang"]
@@ -234,6 +236,8 @@ def get_regexs():
         r"^(// \+build.*\n)+\n", re.MULTILINE)
     # strip //go:build build constraints
     regexs["go_build_constraints_2"] = re.compile(
+        r"^(//go:build.*\n)+\n", re.MULTILINE)
+    regexs["go_build_constraints_3"] = re.compile(
         r"^(//go:build.*\n)", re.MULTILINE)
     # strip #!.* from shell/python scripts
     regexs["shebang"] = re.compile(r"^(#!.*\n)\n*", re.MULTILINE)