[systemtap trunk] Build error on eBPF.

[Tetsuo Handa]

Hello.

Commit 7e2cd9f56293f697 ("Squashed commit of the eBPF backend for stap")
caused following build error when trying to build on CentOS 7.

  CXX      stap-loc2stap.o
loc2stap.cxx: In member function ‘location* location_context::translate(const Dwarf_Op*, size_t, size_t, location*, bool, bool)’:
loc2stap.cxx:239:34: error: missing initializer for member ‘Dwarf_Block::length’ [-Werror=missing-field-initializers]
   Dwarf_Block implicit_value = { };
                                  ^
loc2stap.cxx:239:34: error: missing initializer for member ‘Dwarf_Block::data’ [-Werror=missing-field-initializers]
cc1plus: all warnings being treated as errors
make[2]: *** [stap-loc2stap.o] Error 1
  CXX      stap-bpf-base.o
In file included from bpf-base.cxx:10:0:
bpf-internal.h:20:23: fatal error: linux/bpf.h: No such file or directory
 #include <linux/bpf.h>
                       ^
compilation terminated.
make[2]: *** [stap-bpf-base.o] Error 1
  CXX      stap-bpf-translate.o
In file included from bpf-translate.cxx:10:0:
bpf-internal.h:20:23: fatal error: linux/bpf.h: No such file or directory
 #include <linux/bpf.h>
                       ^
compilation terminated.
make[2]: *** [stap-bpf-translate.o] Error 1
  CXX      stap-bpf-opt.o
In file included from bpf-opt.cxx:15:0:
bpf-internal.h:20:23: fatal error: linux/bpf.h: No such file or directory
 #include <linux/bpf.h>
                       ^
compilation terminated.
make[2]: *** [stap-bpf-opt.o] Error 1