Add constructor to stap_static_probe_ops

Add constructor to stap_static_probe_ops

PR build/24937 concerns an error given by the clang provided by a
particular version of macOS.  In particular, it reports

    error: default initialization of an object of const type 'const
    stap_static_probe_ops' without a user-provided default constructor

Although (at least according to sources I found online) this was
resolved as a bug in the standard, it seemed simple enough to work
around this.

Given that this is a trivial build fix, I think it should go on the
gdb 9 branch as well.

gdb/ChangeLog
2020-01-07  Tom Tromey  <tromey@adacore.com>

	PR build/24937:
	* stap-probe.c (class stap_static_probe_ops): Add constructor.

Change-Id: I18f180c17850f420e9b66afc67f9cb3d8dceb0b3