Include missing header file

This commit was SVN r2083.

src/mca/pcm/base/pcm_base_comm.c

@@ -4,6 +4,8 @@
 
 #include "ompi_config.h"
 
+#include <string.h>
+
 #include "class/ompi_list.h"
 #include "mca/pcm/base/base.h"