buildtools: allow pedantic empty pmdinfo

buildtools: allow pedantic empty pmdinfo

If a driver has no info generated by pmdinfogen,
and if this driver is compiled as pedantic,
then an failure would occur:
	drivers/rte_common_mlx5.pmd.c:1: error:
	ISO C forbids an empty translation unit [-Werror=pedantic]
Such error is triggered with the new mlx5 common directory.

In order to allow an "empty driver info" compiled in pedantic mode,
the script generating .pmd.c file is modified to add a static string
unconditionnaly.
The minimal generated code is:
	static __attribute__((unused)) const char *generator =
		"/path/to/dpdk/buildtools/gen-pmdinfo-cfile.sh";

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>