Doxyfile.in: fix build out-of-tree

Doxyfile.in: fix build out-of-tree

When doxygen is built out-of-tree, it fails to find the generated
file startup_tcl.inc:
	src/openocd.c:59: warning: include file startup_tcl.inc
	not found, perhaps you forgot to add its directory to
	INCLUDE_PATH?

Add '@builddir@/src' to INCLUDE_PATH.

Change-Id: I51f2f6fe7224bba0f8b3db7219f9831de4e67139
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5998
Tested-by: jenkins