Fix CONFIG_WPA_TRACE=y compilation without CONFIG_WPA_TRACE_BFD=y

Fix CONFIG_WPA_TRACE=y compilation without CONFIG_WPA_TRACE_BFD=y

syms is defined only for WPA_TRACE_BFD:

../src/utils/trace.c: In function ‘wpa_trace_deinit’:
../src/utils/trace.c:372:7: error: ‘syms’ undeclared (first use in this function)
  free(syms);
       ^
../src/utils/trace.c:372:7: note: each undeclared identifier is reported only once for each function it appears in
make: *** [../src/utils/trace.o] Error 1
make: *** Waiting for unfinished jobs....

Add appropriate #ifdef.

Signed-off-by: Eliad Peller <eliad@wizery.com>