hash: fix build on IBM POWER and ARM

This patch fixes the compile errors caused by lacking of "size_t"
definition in rte_hash.h.
The compile error exists on IBM POWER and ARM.

The errors are like:
In file included from app/test/test_hash_scaling.c:35:0:
rte_hash.h:70:70: error: unknown type name ‘size_t’

Fixes: 95da2f8e9c ("hash: customize compare function")

Signed-off-by: Chao Zhu <chaozhu@linux.vnet.ibm.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>

lib/librte_hash/rte_hash.h

@@ -41,6 +41,7 @@
  */
 
 #include <stdint.h>
+#include <stddef.h>
 
 #ifdef __cplusplus
 extern "C" {