Fix gdb C++ build when libipt is available

With libipt's headers installed, a build with --enable-build-with-cxx
fails with:

 .../src/gdb/btrace.c: In function ‘btrace_insn_flag pt_btrace_insn_flags(const pt_insn*)’:
 .../src/gdb/btrace.c:734:33: error: invalid conversion from ‘int’ to ‘btrace_insn_flag’ [-fpermissive]
    enum btrace_insn_flag flags = 0;
				  ^
 .../src/gdb/btrace.c:737:11: error: invalid conversion from ‘int’ to ‘btrace_insn_flag’ [-fpermissive]
      flags |= BTRACE_INSN_FLAG_SPECULATIVE;
	    ^

gdb/ChangeLog:
2016-04-15  Pedro Alves  <palves@redhat.com>

	* btrace.c (pt_btrace_insn_flags): Change return type to
	btrace_insn_flags.  Use btrace_insn_flags for local.