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

Doxyfile.in

@@ -600,7 +600,7 @@ RECURSIVE              = YES
 # excluded from the INPUT source files. This way you can easily exclude a
 # subdirectory from a directory tree whose root is specified with the INPUT tag.
 
-EXCLUDE                =
+EXCLUDE                = @srcdir@/src/jtag/drivers/libjaylink
 
 # The EXCLUDE_SYMLINKS tag can be used select whether or not files or
 # directories that are symbolic links (a Unix filesystem feature) are excluded