Enable GCOV profiling on ftrace subsystem

configname: CONFIG_GCOV_PROFILE_FTRACE

Linux Kernel Configuration
└─>Kernel hacking
└─>Tracers
└─>Enable GCOV profiling on ftrace subsystem
In linux kernel since version 4.2 (release Date: 2015-08-30)  
Enable GCOV profiling on ftrace subsystem for checking
which functions/lines are tested.

If unsure, say N.

Note that on a kernel compiled with this config, ftrace will
run significantly slower.