eal: fix C++17 compilation

eal: fix C++17 compilation

Compiling a C++ application that includes directly or indirectly
rte_common.h will cause a warning:

include/rte_common.h:350:37: warning: ISO C++17 does not allow
  ‘register’ storage class specifier [-Wregister]
 rte_combine32ms1b(register uint32_t x)

C++ is pickier than standard C and flags this antique usage.

The register keyword is an old K&R legacy and should be removed
everywhere in DPDK. For now, fix it where it hurts.

Fixes: 08f683174e94 ("eal: add functions for previous power of 2 alignment")
Cc: stable@dpdk.org
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>