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

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-01-07  Tom Tromey  <tromey@adacore.com>
+
+	PR build/24937:
+	* stap-probe.c (class stap_static_probe_ops): Add constructor.
+
 2020-01-02  Jon Turney  <jon.turney@dronecode.org.uk>
 
 	* cli/cli-style.c: Set cli_styling to 'true' in the Cygwin build.

gdb/stap-probe.c

@@ -101,6 +101,12 @@ struct stap_probe_arg
 class stap_static_probe_ops : public static_probe_ops
 {
 public:
+  /* We need a user-provided constructor to placate some compilers.
+     See PR build/24937.  */
+  stap_static_probe_ops ()
+  {
+  }
+
   /* See probe.h.  */
   bool is_linespec (const char **linespecp) const override;