doc: put doxygen log file in build directory
doc: put doxygen log file in build directory

The meson documentation states that projects should not rely upon the
custom_target build commands are run from any given directory.  Therefore,
rather than writing the standout output from doxygen to the current
directory - which could be anywhere in future, put it into the api
directory, so that it is in a known location.

Signed-off-by: Bruce Richardson <66b9e8e8fbd5e9f41f91b0af050876182ce9d0e4@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk