perf: Add LDFLAGS to allow build of old kernels without patching

The LDFLAGS is required or some old kernels fails due missing
symbols and this is preferred than requiring patches to every old
supported kernel.

Fixes [YOCTO: #5221]

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>