doc: fix PDF with greek letter

doc: fix PDF with greek letter

For an unknown reason, the sign "μ" is not accepted by some environments,
probably due to the version of some Latex packages or dependencies.

	! Package inputenc Error: Unicode character μ (U+03BC)
	(inputenc)                not set up for use with LaTeX.

It is fixed by installing texlive-langgreek.

Fixes: d0dff9ba445e ("doc: sample application user guide")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: John McNamara <john.mcnamara@intel.com>