bpf: fix for lex/yacc build error with gcc-5

Fix build error found with Ubuntu shipped gcc-5

~/git/bpf/tools/bpf$ make all

Auto-detecting system features:
...                        libbfd: [ OFF ]
...        disassembler-four-args: [ OFF ]

  CC       bpf_jit_disasm.o
  LINK     bpf_jit_disasm
  CC       bpf_dbg.o
/home/john/git/bpf/tools/bpf/bpf_dbg.c: In function ‘cmd_load’:
/home/john/git/bpf/tools/bpf/bpf_dbg.c:1077:13: warning: ‘cont’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  } else if (matches(subcmd, "pcap") == 0) {
             ^
  LINK     bpf_dbg
  CC       bpf_asm.o
make: *** No rule to make target `bpf_exp.yacc.o', needed by `bpf_asm'.  Stop.

Fixes: 5a8997f2 ("tools: bpf: respect output directory during build")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>