Doxyfile.in: exclude libjaylink from doxygen

Doxyfile.in: exclude libjaylink from doxygen

When build using libjaylink as git submodule, doxygen includes the
libjaylink files and complains for multiple 'mainpage' comment
block, one in OpenOCD and the other in libjaylink:
	src/jtag/drivers/libjaylink/libjaylink/core.c:37: warning:
	found more than one \mainpage comment block! (first
	occurrence: doc/manual/main.txt, line 1), Skipping current
	block!

Exclude libjaylink submodule from doxygen.

Change-Id: I5e856817344c9f21f8c26f077a23c00b83cfbcb5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6000
Tested-by: jenkins