define CC_USING_FENTRY if CONFIG_FUNCTION_TRACER is defined
define CC_USING_FENTRY if CONFIG_FUNCTION_TRACER is defined

Fix issue #2734.

In 4.18 and later, when CONFIG_FUNCTION_TRACER is defined, kernel Makefile adds
-DCC_USING_FENTRY. Let do the same for bpf programs.

Signed-off-by: Yonghong Song <e76892bbe783db2f627de1232e139db4de43da60@fb.com>

brendangregg/bcc,iovisor/bcc,iovisor/bcc,brendangregg/bcc,tuxology/bcc,brendangregg/bcc,tuxology/bcc,tuxology/bcc,iovisor/bcc,tuxology/bcc,iovisor/bcc,brendangregg/bcc,tuxology/bcc,brendangregg/bcc,iovisor/bcc