kexecboot_0.6: fix build with gcc 5

Resolving the issue:

rgb.h:55:1: warning: inline function 'rgba2comp' declared but never defined
|  rgba2comp(kx_rgba rgba, kx_ccomp *red, kx_ccomp *green,
|  ^

Fix C99 inline semantic as documented here:
https://gcc.gnu.org/gcc-5/porting_to.html



Signed-off-by: Andrea Adami <andrea.adami@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>