perf jvmti: Fix linker error when libelf config is disabled

perf jvmti: Fix linker error when libelf config is disabled

When libelf is disabled in the configuration, we get the following
linker error:

  LINK     libperf-jvmti.so
  ld: cannot find -lelf
  Makefile.perf:515: recipe for target 'libperf-jvmti.so' failed

Jiri pointed out that both librt and libelf are not really required. So
this patch fixes the linker error by getting rid of unwanted libraries
in the linker stage.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: David Carrillo-Cisneros <davidcc@google.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Elena Reshetova <elena.reshetova@intel.com>
Cc: Kees Kook <keescook@chromium.org>
Cc: Paul Turner <pjt@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 209045adc2bb ("perf tools: add JVMTI agent library")
Link: http://lkml.kernel.org/r/20170719011839.99399-5-davidcc@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>