support: Include <string.h> for strcmp in support_format_addrinfo.c

This is currently implied by the internal headers, but it makes
sense not to rely on this.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

support/support_format_addrinfo.c

@@ -22,6 +22,7 @@
 #include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <support/support.h>
 #include <support/xmemstream.h>