Fix compile error with clang 3.8

Fix compile error with clang 3.8

When compiling with clang 3.8 (default clang version on Debian
Stretch, the current stable), we get errors like this:

  CXX    dtrace-probe.o
../../binutils-gdb/gdb/dtrace-probe.c:103:31: error: default initialization of an object of const type 'const dtrace_static_probe_ops' without a user-provided default constructor
const dtrace_static_probe_ops dtrace_static_probe_ops;
                              ^

Silence them by value-initializing those objects.  It's not necessary
with other compilers (later clang versions, gcc), but it shouldn't
hurt either.